Add domain context to Toon meta; IId ref tests for JSON/Bin

- Support optional domain context in Toon serializer metadata (@meta section now includes "context" property if provided).
- Update internal and public APIs to accept and propagate domain context.
- Add overloads for Serialize, SerializeTypeMetadata, and SerializeMetadata to allow specifying domain context.
- Add AcBinarySerializerIIdReferenceTests and AcJsonSerializerIIdReferenceTests to verify IId-based reference deduplication and reference identity for both binary and JSON serializers.
- Tests cover both int and Guid IId scenarios, data integrity, and output size efficiency.

AyCode.Core.Tests/Serialization/AcBinarySerializerIIdReferenceTests.cs

@@ -0,0 +1,468 @@
+using AyCode.Core.Extensions;
+using AyCode.Core.Serializers.Binaries;
+using AyCode.Core.Tests.TestModels;
+
+namespace AyCode.Core.Tests.Serialization;
+
+/// <summary>
+/// Tests for IId-based reference handling in Binary serializer.
+/// Two scenarios:
+/// 1. Same instance referenced multiple times (object identity)
+/// 2. Different instances with same IId.Id (IId-based deduplication)
+/// 
+/// Tests verify BOTH:
+/// - Serialized output uses ObjectRef (not redundant full objects)
+/// - Deserialized result maintains reference identity
+/// </summary>
+[TestClass]
+public class AcBinarySerializerIIdReferenceTests
+{
+    // BinaryTypeCode.ObjectRef = 27
+    private const byte ObjectRefTypeCode = 27;
+
+    #region Helper Methods
+
+    /// <summary>
+    /// Counts occurrences of ObjectRef (0x1B = 27) in binary data.
+    /// </summary>
+    private static int CountObjectRefs(byte[] binary)
+    {
+        var count = 0;
+        for (var i = 0; i < binary.Length; i++)
+        {
+            if (binary[i] == ObjectRefTypeCode)
+                count++;
+        }
+        return count;
+    }
+
+    /// <summary>
+    /// Counts occurrences of a string in binary data (UTF8).
+    /// </summary>
+    private static int CountStringOccurrences(byte[] binary, string searchString)
+    {
+        var searchBytes = System.Text.Encoding.UTF8.GetBytes(searchString);
+        var count = 0;
+        for (var i = 0; i <= binary.Length - searchBytes.Length; i++)
+        {
+            var match = true;
+            for (var j = 0; j < searchBytes.Length; j++)
+            {
+                if (binary[i + j] != searchBytes[j])
+                {
+                    match = false;
+                    break;
+                }
+            }
+            if (match) count++;
+        }
+        return count;
+    }
+
+    #endregion
+
+    #region Scenario 1: Same Instance (Object Identity)
+
+    /// <summary>
+    /// SCENARIO 1: Same instance referenced multiple times.
+    /// Validates: ObjectRef present + data integrity after deserialize + reference identity.
+    /// </summary>
+    [TestMethod]
+    public void SameInstance_SerializeAndDeserialize()
+    {
+        // Arrange: SAME instance used 4 times
+        var sharedTag = new SharedTag { Id = 1, Name = "ImportantTag", Color = "#FF0000" };
+        
+        var order = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "ORD-001",
+            PrimaryTag = sharedTag,
+            Items =
+            [
+                new TestOrderItem { Id = 1, ProductName = "Product-A", Tag = sharedTag },
+                new TestOrderItem { Id = 2, ProductName = "Product-B", Tag = sharedTag },
+                new TestOrderItem { Id = 3, ProductName = "Product-C", Tag = sharedTag }
+            ]
+        };
+
+        // Act
+        var binary = order.ToBinary();
+        var result = binary.BinaryTo<TestOrder>();
+
+        // Assert 1: Binary should have ObjectRef entries (reference handling is active)
+        var objectRefCount = CountObjectRefs(binary);
+        Console.WriteLine($"Binary size: {binary.Length} bytes");
+        Console.WriteLine($"ObjectRef count: {objectRefCount}");
+        
+        Assert.IsTrue(objectRefCount >= 3, 
+            $"Expected at least 3 ObjectRef entries for shared tag, found {objectRefCount}. " +
+            "Reference handling may not be active!");
+
+        // Assert 2: Data integrity - ALL tags are present and have correct data
+        Assert.IsNotNull(result, "Deserialized result is null");
+        Assert.IsNotNull(result.PrimaryTag, "PrimaryTag is null - data lost!");
+        Assert.AreEqual(1, result.PrimaryTag.Id, "PrimaryTag.Id incorrect");
+        Assert.AreEqual("ImportantTag", result.PrimaryTag.Name, "PrimaryTag.Name incorrect");
+        Assert.AreEqual("#FF0000", result.PrimaryTag.Color, "PrimaryTag.Color incorrect");
+        
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            Assert.IsNotNull(result.Items[i].Tag, $"Items[{i}].Tag is null - data lost!");
+            Assert.AreEqual(1, result.Items[i].Tag!.Id, $"Items[{i}].Tag.Id incorrect");
+            Assert.AreEqual("ImportantTag", result.Items[i].Tag.Name, $"Items[{i}].Tag.Name incorrect");
+        }
+
+        // Assert 3: Reference identity - all should be same object reference
+        Assert.AreSame(result.PrimaryTag, result.Items[0].Tag, 
+            "Item[0].Tag should be same reference as PrimaryTag");
+        Assert.AreSame(result.PrimaryTag, result.Items[1].Tag,
+            "Item[1].Tag should be same reference as PrimaryTag");
+        Assert.AreSame(result.PrimaryTag, result.Items[2].Tag,
+            "Item[2].Tag should be same reference as PrimaryTag");
+    }
+
+    #endregion
+
+    #region Scenario 2: Different Instances with Same IId (IId-Based Deduplication)
+
+    /// <summary>
+    /// SCENARIO 2: DIFFERENT instances with SAME IId.Id value.
+    /// CRITICAL test - if IId-based deduplication works:
+    /// - ObjectRef should be used in binary
+    /// - Data should be complete after deserialize
+    /// - References should be identical (AreSame)
+    /// - Different TYPES with same int Id should NOT be confused!
+    /// </summary>
+    [TestMethod]
+    public void DifferentInstances_SameIId_SerializeAndDeserialize()
+    {
+        // Arrange: DIFFERENT instances but SAME IId.Id
+        // CRITICAL: Multiple DIFFERENT TYPES all have Id=1 - must not be confused!
+        var order = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "ORD-001",
+            // All three types have Id=1 - tests (Type, Id) keying, not just Id
+            PrimaryTag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+            Owner = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" },
+            Category = new SharedCategory { Id = 1, Name = "Category_Id1", SortOrder = 10 },
+            Items =
+            [
+                new TestOrderItem 
+                { 
+                    Id = 1, 
+                    ProductName = "Product-A", 
+                    Tag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+                    Assignee = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" }
+                },
+                new TestOrderItem 
+                { 
+                    Id = 2, 
+                    ProductName = "Product-B", 
+                    Tag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+                    Assignee = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" }
+                },
+                new TestOrderItem 
+                { 
+                    Id = 3, 
+                    ProductName = "Product-C", 
+                    Tag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+                    Assignee = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" }
+                }
+            ]
+        };
+
+        // Act
+        var binary = order.ToBinary();
+        var result = binary.BinaryTo<TestOrder>();
+
+        // Assert 1: Check if ObjectRef is used (IId-based deduplication active)
+        var objectRefCount = CountObjectRefs(binary);
+        Console.WriteLine($"Binary size: {binary.Length} bytes");
+        Console.WriteLine($"ObjectRef count: {objectRefCount}");
+        
+        // 4 Tags, 4 Users - each should have 3 ObjectRefs = 6 total minimum
+        Assert.IsTrue(objectRefCount >= 6, 
+            $"CRITICAL: Expected at least 6 ObjectRef entries (3 per type for Tag and User), found {objectRefCount}. " +
+            "IId-based reference deduplication is NOT working!");
+
+        // Assert 2: Data integrity - ALL data present and correct
+        Assert.IsNotNull(result, "Deserialized result is null");
+        
+        // Tag data
+        Assert.IsNotNull(result.PrimaryTag, "PrimaryTag is null - data lost!");
+        Assert.AreEqual(1, result.PrimaryTag.Id, "PrimaryTag.Id incorrect");
+        Assert.AreEqual("Tag_Id1", result.PrimaryTag.Name, "PrimaryTag.Name incorrect - might be confused with User!");
+        Assert.AreEqual("#FF0000", result.PrimaryTag.Color, "PrimaryTag.Color incorrect");
+        
+        // User data - MUST NOT be confused with Tag (both have Id=1)
+        Assert.IsNotNull(result.Owner, "Owner is null - data lost!");
+        Assert.AreEqual(1, result.Owner.Id, "Owner.Id incorrect");
+        Assert.AreEqual("User_Id1", result.Owner.Username, "Owner.Username incorrect - might be confused with Tag!");
+        Assert.AreEqual("user1@test.com", result.Owner.Email, "Owner.Email incorrect");
+        
+        // Category data - MUST NOT be confused with Tag or User (all have Id=1)
+        Assert.IsNotNull(result.Category, "Category is null - data lost!");
+        Assert.AreEqual(1, result.Category.Id, "Category.Id incorrect");
+        Assert.AreEqual("Category_Id1", result.Category.Name, "Category.Name incorrect - might be confused with Tag!");
+        Assert.AreEqual(10, result.Category.SortOrder, "Category.SortOrder incorrect");
+        
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            // Tag in items
+            Assert.IsNotNull(result.Items[i].Tag, $"Items[{i}].Tag is null - data lost!");
+            Assert.AreEqual(1, result.Items[i].Tag!.Id, $"Items[{i}].Tag.Id incorrect");
+            Assert.AreEqual("Tag_Id1", result.Items[i].Tag.Name, $"Items[{i}].Tag.Name incorrect - confused with User?");
+            
+            // User in items - MUST NOT be confused with Tag
+            Assert.IsNotNull(result.Items[i].Assignee, $"Items[{i}].Assignee is null - data lost!");
+            Assert.AreEqual(1, result.Items[i].Assignee!.Id, $"Items[{i}].Assignee.Id incorrect");
+            Assert.AreEqual("User_Id1", result.Items[i].Assignee.Username, $"Items[{i}].Assignee.Username incorrect - confused with Tag?");
+        }
+
+        // Assert 3: Reference identity - same TYPE with same Id should be same reference
+        // Tags with Id=1 should all be same reference
+        Assert.AreSame(result.PrimaryTag, result.Items[0].Tag, 
+            "CRITICAL: Item[0].Tag should be same reference as PrimaryTag (same SharedTag.Id=1)");
+        Assert.AreSame(result.PrimaryTag, result.Items[1].Tag,
+            "CRITICAL: Item[1].Tag should be same reference as PrimaryTag (same SharedTag.Id=1)");
+        Assert.AreSame(result.PrimaryTag, result.Items[2].Tag,
+            "CRITICAL: Item[2].Tag should be same reference as PrimaryTag (same SharedTag.Id=1)");
+        
+        // Users with Id=1 should all be same reference
+        Assert.AreSame(result.Owner, result.Items[0].Assignee,
+            "CRITICAL: Item[0].Assignee should be same reference as Owner (same SharedUser.Id=1)");
+        Assert.AreSame(result.Owner, result.Items[1].Assignee,
+            "CRITICAL: Item[1].Assignee should be same reference as Owner (same SharedUser.Id=1)");
+        Assert.AreSame(result.Owner, result.Items[2].Assignee,
+            "CRITICAL: Item[2].Assignee should be same reference as Owner (same SharedUser.Id=1)");
+        
+        // Assert 4: Different TYPES with same Id should NOT be same reference!
+        Assert.AreNotSame<object>(result.PrimaryTag, result.Owner,
+            "CRITICAL BUG: Tag and User are same reference! Types with same int Id were confused!");
+        Assert.AreNotSame<object>(result.PrimaryTag, result.Category,
+            "CRITICAL BUG: Tag and Category are same reference! Types with same int Id were confused!");
+        Assert.AreNotSame<object>(result.Owner, result.Category,
+            "CRITICAL BUG: User and Category are same reference! Types with same int Id were confused!");
+    }
+
+    /// <summary>
+    /// Size comparison: Same IId should result in smaller binary + verify data integrity.
+    /// </summary>
+    [TestMethod]
+    public void DifferentInstances_SameIId_SmallerBinaryWithDataIntegrity()
+    {
+        // Arrange: 10 different instances with SAME IId
+        var orderWithSameIId = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "SAME-IID",
+            Items = Enumerable.Range(1, 10).Select(i => new TestOrderItem
+            {
+                Id = i,
+                ProductName = $"Product-{i}",
+                // All have SAME IId.Id = 1, but DIFFERENT instances
+                Assignee = new SharedUser { Id = 1, Username = "shared_user_name", Email = "shared@test.com" }
+            }).ToList()
+        };
+
+        // Arrange: 10 different instances with DIFFERENT IIds
+        var orderWithDifferentIIds = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "DIFF-IID",
+            Items = Enumerable.Range(1, 10).Select(i => new TestOrderItem
+            {
+                Id = i,
+                ProductName = $"Product-{i}",
+                // All have DIFFERENT IId.Id
+                Assignee = new SharedUser { Id = i * 100, Username = "unique_user_name", Email = "unique@test.com" }
+            }).ToList()
+        };
+
+        // Act
+        var sameIIdBinary = orderWithSameIId.ToBinary();
+        var diffIIdBinary = orderWithDifferentIIds.ToBinary();
+        var sameIIdResult = sameIIdBinary.BinaryTo<TestOrder>();
+        var diffIIdResult = diffIIdBinary.BinaryTo<TestOrder>();
+
+        // Assert 1: Size comparison
+        Console.WriteLine($"Same IId binary size: {sameIIdBinary.Length} bytes");
+        Console.WriteLine($"Different IId binary size: {diffIIdBinary.Length} bytes");
+        Console.WriteLine($"Size difference: {diffIIdBinary.Length - sameIIdBinary.Length} bytes");
+        
+        Assert.IsTrue(sameIIdBinary.Length < diffIIdBinary.Length,
+            $"Same IId ({sameIIdBinary.Length}b) should be smaller than different IIds ({diffIIdBinary.Length}b). " +
+            "IId-based deduplication NOT working!");
+
+        // Assert 2: Data integrity for sameIId result
+        Assert.IsNotNull(sameIIdResult, "sameIIdResult is null");
+        Assert.IsNotNull(sameIIdResult.Items, "sameIIdResult.Items is null");
+        Assert.AreEqual(10, sameIIdResult.Items.Count, "sameIIdResult should have 10 items");
+        
+        for (var i = 0; i < 10; i++)
+        {
+            Assert.IsNotNull(sameIIdResult.Items[i].Assignee, 
+                $"sameIIdResult.Items[{i}].Assignee is null - data lost!");
+            Assert.AreEqual(1, sameIIdResult.Items[i].Assignee!.Id,
+                $"sameIIdResult.Items[{i}].Assignee.Id should be 1");
+            Assert.AreEqual("shared_user_name", sameIIdResult.Items[i].Assignee.Username,
+                $"sameIIdResult.Items[{i}].Assignee.Username incorrect");
+        }
+
+        // Assert 3: Reference identity for sameIId
+        var firstAssignee = sameIIdResult.Items[0].Assignee;
+        for (var i = 1; i < 10; i++)
+        {
+            Assert.AreSame(firstAssignee, sameIIdResult.Items[i].Assignee,
+                $"CRITICAL: Items[{i}].Assignee should be same reference as Items[0].Assignee (same IId.Id=1)");
+        }
+    }
+
+    #endregion
+
+    #region Guid-Based IId Tests
+
+    /// <summary>
+    /// Guid IId with same instance - validates ObjectRef + data integrity + reference identity.
+    /// </summary>
+    [TestMethod]
+    public void GuidIId_SameInstance_SerializeAndDeserialize()
+    {
+        // Arrange: SAME instance
+        var sharedGuid = Guid.NewGuid();
+        var sharedItem = new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" };
+        
+        var order = new TestGuidOrder
+        {
+            Id = Guid.NewGuid(),
+            Code = "GUID-001",
+            Count = 3,
+            Items = [sharedItem, sharedItem, sharedItem]
+        };
+
+        // Act
+        var binary = order.ToBinary();
+        var result = binary.BinaryTo<TestGuidOrder>();
+
+        // Assert 1: ObjectRef should be present
+        var objectRefCount = CountObjectRefs(binary);
+        Console.WriteLine($"Binary size: {binary.Length} bytes");
+        Console.WriteLine($"ObjectRef count: {objectRefCount}");
+        
+        Assert.IsTrue(objectRefCount >= 2, $"Expected at least 2 ObjectRefs, found {objectRefCount}");
+
+        // Assert 2: Data integrity - all items present and correct
+        Assert.IsNotNull(result, "Result is null");
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            Assert.IsNotNull(result.Items[i], $"Items[{i}] is null");
+            Assert.AreEqual(sharedGuid, result.Items[i].Id, $"Items[{i}].Id incorrect");
+            Assert.AreEqual("SharedGuidItem", result.Items[i].Name, $"Items[{i}].Name incorrect");
+        }
+
+        // Assert 3: Reference identity
+        Assert.AreSame(result.Items[0], result.Items[1], "Items[0] and Items[1] should be same reference");
+        Assert.AreSame(result.Items[1], result.Items[2], "Items[1] and Items[2] should be same reference");
+    }
+
+    /// <summary>
+    /// CRITICAL: Guid IId with different instances but same Id - tests IId-based deduplication.
+    /// </summary>
+    [TestMethod]
+    public void GuidIId_DifferentInstances_SameId_SerializeAndDeserialize()
+    {
+        // Arrange: DIFFERENT instances but SAME Guid
+        var sharedGuid = Guid.NewGuid();
+        
+        var order = new TestGuidOrder
+        {
+            Id = Guid.NewGuid(),
+            Code = "GUID-001",
+            Count = 3,
+            Items =
+            [
+                new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" },
+                new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" },
+                new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" }
+            ]
+        };
+
+        // Act
+        var binary = order.ToBinary();
+        var result = binary.BinaryTo<TestGuidOrder>();
+
+        // Assert 1: ObjectRef should be present if IId-based dedup works
+        var objectRefCount = CountObjectRefs(binary);
+        Console.WriteLine($"Binary size: {binary.Length} bytes");
+        Console.WriteLine($"ObjectRef count: {objectRefCount}");
+        
+        Assert.IsTrue(objectRefCount >= 2, 
+            $"CRITICAL: Expected at least 2 ObjectRefs for same Guid IId, found {objectRefCount}. " +
+            "Guid-based IId deduplication NOT working!");
+
+        // Assert 2: Data integrity - all items present and correct
+        Assert.IsNotNull(result, "Result is null");
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            Assert.IsNotNull(result.Items[i], $"Items[{i}] is null - data lost!");
+            Assert.AreEqual(sharedGuid, result.Items[i].Id, $"Items[{i}].Id incorrect");
+            Assert.AreEqual("SharedGuidItem", result.Items[i].Name, $"Items[{i}].Name incorrect");
+        }
+
+        // Assert 3: Reference identity - if IId-based, should be same reference
+        Assert.AreSame(result.Items[0], result.Items[1], 
+            "CRITICAL: Items[0] and Items[1] should be same reference (same Guid)");
+        Assert.AreSame(result.Items[1], result.Items[2], 
+            "CRITICAL: Items[1] and Items[2] should be same reference (same Guid)");
+    }
+
+    #endregion
+
+    #region Data Integrity Tests
+
+    /// <summary>
+    /// Verify data is correct regardless of reference handling.
+    /// </summary>
+    [TestMethod]
+    public void SharedCategory_DataIntegrity()
+    {
+        var categories = new List<SharedCategory>
+        {
+            new() { Id = 1, Name = "Category1", SortOrder = 1, IsDefault = true },
+            new() { Id = 2, Name = "Category2", SortOrder = 2, ParentCategoryId = 1 },
+            new() { Id = 3, Name = "Category3", SortOrder = 3, ParentCategoryId = 1 }
+        };
+
+        var binary = categories.ToBinary();
+        var result = binary.BinaryTo<List<SharedCategory>>();
+
+        Assert.IsNotNull(result);
+        Assert.AreEqual(3, result.Count);
+        
+        Assert.AreEqual(1, result[0].Id);
+        Assert.AreEqual("Category1", result[0].Name);
+        Assert.IsTrue(result[0].IsDefault);
+        
+        Assert.AreEqual(2, result[1].Id);
+        Assert.AreEqual(1, result[1].ParentCategoryId);
+        
+        Assert.AreEqual(3, result[2].Id);
+        Assert.AreEqual(1, result[2].ParentCategoryId);
+    }
+
+    #endregion
+}

AyCode.Core.Tests/Serialization/AcJsonSerializerIIdReferenceTests.cs

@@ -0,0 +1,441 @@
+using AyCode.Core.Extensions;
+using AyCode.Core.Tests.TestModels;
+
+namespace AyCode.Core.Tests.Serialization;
+
+/// <summary>
+/// Tests for IId-based reference handling in JSON serializer.
+/// Two scenarios:
+/// 1. Same instance referenced multiple times (object identity)
+/// 2. Different instances with same IId.Id (IId-based deduplication)
+/// 
+/// Tests verify BOTH:
+/// - Serialized JSON uses $ref (not redundant full objects)
+/// - Deserialized result maintains reference identity
+/// </summary>
+[TestClass]
+public class AcJsonSerializerIIdReferenceTests
+{
+    #region Helper Methods
+
+    /// <summary>
+    /// Counts occurrences of a string in JSON.
+    /// </summary>
+    private static int CountOccurrences(string json, string searchString)
+    {
+        var count = 0;
+        var index = 0;
+        while ((index = json.IndexOf(searchString, index, StringComparison.Ordinal)) != -1)
+        {
+            count++;
+            index += searchString.Length;
+        }
+        return count;
+    }
+
+    #endregion
+
+    #region Scenario 1: Same Instance (Object Identity)
+
+    /// <summary>
+    /// SCENARIO 1: Same instance referenced multiple times.
+    /// JSON should contain $ref for 2nd, 3rd, 4th occurrence.
+    /// Validates: $ref present + data integrity after deserialize + reference identity
+    /// </summary>
+    [TestMethod]
+    public void SameInstance_Json_SerializeAndDeserialize()
+    {
+        // Arrange: SAME instance used 4 times
+        var sharedTag = new SharedTag { Id = 1, Name = "ImportantTag", Color = "#FF0000" };
+        
+        var order = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "ORD-001",
+            PrimaryTag = sharedTag,
+            Items =
+            [
+                new TestOrderItem { Id = 1, ProductName = "Product-A", Tag = sharedTag },
+                new TestOrderItem { Id = 2, ProductName = "Product-B", Tag = sharedTag },
+                new TestOrderItem { Id = 3, ProductName = "Product-C", Tag = sharedTag }
+            ]
+        };
+
+        // Act
+        var json = order.ToJson();
+        Console.WriteLine(json);
+        var result = json.JsonTo<TestOrder>();
+
+        // Assert 1: JSON contains $ref markers (reference handling is active)
+        var refCount = CountOccurrences(json, "{\"$ref\":\"1\"}");
+        Console.WriteLine($"JSON length: {json.Length} chars");
+        Console.WriteLine($"$ref count: {refCount}");
+        
+        Assert.IsTrue(refCount >= 3, 
+            $"Expected at least 3 $ref entries for shared tag, found {refCount}. " +
+            "Reference handling may not be active!");
+
+        // Assert 2: Data integrity - ALL tags are present and have correct data
+        Assert.IsNotNull(result, "Deserialized result is null");
+        Assert.IsNotNull(result.PrimaryTag, "PrimaryTag is null - data lost!");
+        Assert.AreEqual(1, result.PrimaryTag.Id, "PrimaryTag.Id incorrect");
+        Assert.AreEqual("ImportantTag", result.PrimaryTag.Name, "PrimaryTag.Name incorrect");
+        Assert.AreEqual("#FF0000", result.PrimaryTag.Color, "PrimaryTag.Color incorrect");
+        
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            Assert.IsNotNull(result.Items[i].Tag, $"Items[{i}].Tag is null - data lost!");
+            Assert.AreEqual(1, result.Items[i].Tag!.Id, $"Items[{i}].Tag.Id incorrect");
+            Assert.AreEqual("ImportantTag", result.Items[i].Tag.Name, $"Items[{i}].Tag.Name incorrect");
+        }
+
+        // Assert 3: Reference identity - all should be same object reference
+        Assert.AreSame(result.PrimaryTag, result.Items[0].Tag, 
+            "Item[0].Tag should be same reference as PrimaryTag");
+        Assert.AreSame(result.PrimaryTag, result.Items[1].Tag,
+            "Item[1].Tag should be same reference as PrimaryTag");
+        Assert.AreSame(result.PrimaryTag, result.Items[2].Tag,
+            "Item[2].Tag should be same reference as PrimaryTag");
+    }
+
+    #endregion
+
+    #region Scenario 2: Different Instances with Same IId (IId-Based Deduplication)
+
+    /// <summary>
+    /// SCENARIO 2: DIFFERENT instances with SAME IId.Id value.
+    /// CRITICAL test - if IId-based deduplication works:
+    /// - $ref should be used in JSON
+    /// - Data should be complete after deserialize
+    /// - References should be identical (AreSame)
+    /// - Different TYPES with same int Id should NOT be confused!
+    /// </summary>
+    [TestMethod]
+    public void DifferentInstances_SameIId_SerializeAndDeserialize()
+    {
+        // Arrange: DIFFERENT instances but SAME IId.Id
+        // CRITICAL: Multiple DIFFERENT TYPES all have Id=1 - must not be confused!
+        var order = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "ORD-001",
+            // All three types have Id=1 - tests (Type, Id) keying, not just Id
+            PrimaryTag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+            Owner = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" },
+            Category = new SharedCategory { Id = 1, Name = "Category_Id1", SortOrder = 10 },
+            Items =
+            [
+                new TestOrderItem 
+                { 
+                    Id = 1, 
+                    ProductName = "Product-A", 
+                    Tag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+                    Assignee = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" }
+                },
+                new TestOrderItem 
+                { 
+                    Id = 2, 
+                    ProductName = "Product-B", 
+                    Tag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+                    Assignee = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" }
+                },
+                new TestOrderItem 
+                { 
+                    Id = 3, 
+                    ProductName = "Product-C", 
+                    Tag = new SharedTag { Id = 1, Name = "Tag_Id1", Color = "#FF0000" },
+                    Assignee = new SharedUser { Id = 1, Username = "User_Id1", Email = "user1@test.com" }
+                }
+            ]
+        };
+
+        // Act
+        var json = order.ToJson();
+        var result = json.JsonTo<TestOrder>();
+
+        // Assert 1: Check if $ref is used (IId-based deduplication active)
+        var refCount = CountOccurrences(json, "\"$ref\"");
+        Console.WriteLine($"JSON length: {json.Length} chars");
+        Console.WriteLine($"$ref count: {refCount}");
+        Console.WriteLine("JSON (first 2000 chars):");
+        Console.WriteLine(json.Length > 2000 ? json[..2000] + "..." : json);
+        
+        // 4 Tags, 4 Users - each should have 3 $refs = 6 total minimum
+        Assert.IsTrue(refCount >= 6, 
+            $"CRITICAL: Expected at least 6 $ref entries (3 per type for Tag and User), found {refCount}. " +
+            "IId-based reference deduplication is NOT working!");
+
+        // Assert 2: Data integrity - ALL data present and correct
+        Assert.IsNotNull(result, "Deserialized result is null");
+        
+        // Tag data
+        Assert.IsNotNull(result.PrimaryTag, "PrimaryTag is null - data lost!");
+        Assert.AreEqual(1, result.PrimaryTag.Id, "PrimaryTag.Id incorrect");
+        Assert.AreEqual("Tag_Id1", result.PrimaryTag.Name, "PrimaryTag.Name incorrect - might be confused with User!");
+        Assert.AreEqual("#FF0000", result.PrimaryTag.Color, "PrimaryTag.Color incorrect");
+        
+        // User data - MUST NOT be confused with Tag (both have Id=1)
+        Assert.IsNotNull(result.Owner, "Owner is null - data lost!");
+        Assert.AreEqual(1, result.Owner.Id, "Owner.Id incorrect");
+        Assert.AreEqual("User_Id1", result.Owner.Username, "Owner.Username incorrect - might be confused with Tag!");
+        Assert.AreEqual("user1@test.com", result.Owner.Email, "Owner.Email incorrect");
+        
+        // Category data - MUST NOT be confused with Tag or User (all have Id=1)
+        Assert.IsNotNull(result.Category, "Category is null - data lost!");
+        Assert.AreEqual(1, result.Category.Id, "Category.Id incorrect");
+        Assert.AreEqual("Category_Id1", result.Category.Name, "Category.Name incorrect - might be confused with Tag!");
+        Assert.AreEqual(10, result.Category.SortOrder, "Category.SortOrder incorrect");
+        
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            // Tag in items
+            Assert.IsNotNull(result.Items[i].Tag, $"Items[{i}].Tag is null - data lost!");
+            Assert.AreEqual(1, result.Items[i].Tag!.Id, $"Items[{i}].Tag.Id incorrect");
+            Assert.AreEqual("Tag_Id1", result.Items[i].Tag.Name, $"Items[{i}].Tag.Name incorrect - confused with User?");
+            
+            // User in items - MUST NOT be confused with Tag
+            Assert.IsNotNull(result.Items[i].Assignee, $"Items[{i}].Assignee is null - data lost!");
+            Assert.AreEqual(1, result.Items[i].Assignee!.Id, $"Items[{i}].Assignee.Id incorrect");
+            Assert.AreEqual("User_Id1", result.Items[i].Assignee.Username, $"Items[{i}].Assignee.Username incorrect - confused with Tag?");
+        }
+
+        // Assert 3: Reference identity - same TYPE with same Id should be same reference
+        // Tags with Id=1 should all be same reference
+        Assert.AreSame(result.PrimaryTag, result.Items[0].Tag, 
+            "CRITICAL: Item[0].Tag should be same reference as PrimaryTag (same SharedTag.Id=1)");
+        Assert.AreSame(result.PrimaryTag, result.Items[1].Tag,
+            "CRITICAL: Item[1].Tag should be same reference as PrimaryTag (same SharedTag.Id=1)");
+        Assert.AreSame(result.PrimaryTag, result.Items[2].Tag,
+            "CRITICAL: Item[2].Tag should be same reference as PrimaryTag (same SharedTag.Id=1)");
+        
+        // Users with Id=1 should all be same reference
+        Assert.AreSame(result.Owner, result.Items[0].Assignee,
+            "CRITICAL: Item[0].Assignee should be same reference as Owner (same SharedUser.Id=1)");
+        Assert.AreSame(result.Owner, result.Items[1].Assignee,
+            "CRITICAL: Item[1].Assignee should be same reference as Owner (same SharedUser.Id=1)");
+        Assert.AreSame(result.Owner, result.Items[2].Assignee,
+            "CRITICAL: Item[2].Assignee should be same reference as Owner (same SharedUser.Id=1)");
+        
+        // Assert 4: Different TYPES with same Id should NOT be same reference!
+        Assert.AreNotSame<object>(result.PrimaryTag, result.Owner,
+            "CRITICAL BUG: Tag and User are same reference! Types with same int Id were confused!");
+        Assert.AreNotSame<object>(result.PrimaryTag, result.Category,
+            "CRITICAL BUG: Tag and Category are same reference! Types with same int Id were confused!");
+        Assert.AreNotSame<object>(result.Owner, result.Category,
+            "CRITICAL BUG: User and Category are same reference! Types with same int Id were confused!");
+    }
+
+    /// <summary>
+    /// Size comparison: Same IId should result in smaller JSON + verify data integrity.
+    /// </summary>
+    [TestMethod]
+    public void DifferentInstances_SameIId_SmallerJsonWithDataIntegrity()
+    {
+        // Arrange: 10 different instances with SAME IId
+        var orderWithSameIId = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "SAME-IID",
+            Items = Enumerable.Range(1, 10).Select(i => new TestOrderItem
+            {
+                Id = i,
+                ProductName = $"Product-{i}",
+                Assignee = new SharedUser { Id = 1, Username = "shared_user_name", Email = "shared@test.com" }
+            }).ToList()
+        };
+
+        // Arrange: 10 different instances with DIFFERENT IIds
+        var orderWithDifferentIIds = new TestOrder
+        {
+            Id = 1,
+            OrderNumber = "DIFF-IID",
+            Items = Enumerable.Range(1, 10).Select(i => new TestOrderItem
+            {
+                Id = i,
+                ProductName = $"Product-{i}",
+                Assignee = new SharedUser { Id = i * 100, Username = "unique_user_name", Email = "unique@test.com" }
+            }).ToList()
+        };
+
+        // Act
+        var sameIIdJson = orderWithSameIId.ToJson();
+        var diffIIdJson = orderWithDifferentIIds.ToJson();
+        var sameIIdResult = sameIIdJson.JsonTo<TestOrder>();
+        var diffIIdResult = diffIIdJson.JsonTo<TestOrder>();
+
+        // Assert 1: Size comparison
+        Console.WriteLine($"Same IId JSON size: {sameIIdJson.Length} chars");
+        Console.WriteLine($"Different IId JSON size: {diffIIdJson.Length} chars");
+        Console.WriteLine($"Size difference: {diffIIdJson.Length - sameIIdJson.Length} chars");
+        
+        Assert.IsTrue(sameIIdJson.Length < diffIIdJson.Length,
+            $"Same IId ({sameIIdJson.Length}) should be smaller than different IIds ({diffIIdJson.Length}). " +
+            "IId-based deduplication NOT working!");
+
+        // Assert 2: Data integrity for sameIId result
+        Assert.IsNotNull(sameIIdResult, "sameIIdResult is null");
+        Assert.IsNotNull(sameIIdResult.Items, "sameIIdResult.Items is null");
+        Assert.AreEqual(10, sameIIdResult.Items.Count, "sameIIdResult should have 10 items");
+        
+        for (var i = 0; i < 10; i++)
+        {
+            Assert.IsNotNull(sameIIdResult.Items[i].Assignee, 
+                $"sameIIdResult.Items[{i}].Assignee is null - data lost!");
+            Assert.AreEqual(1, sameIIdResult.Items[i].Assignee!.Id,
+                $"sameIIdResult.Items[{i}].Assignee.Id should be 1");
+            Assert.AreEqual("shared_user_name", sameIIdResult.Items[i].Assignee.Username,
+                $"sameIIdResult.Items[{i}].Assignee.Username incorrect");
+        }
+
+        // Assert 3: Reference identity for sameIId
+        var firstAssignee = sameIIdResult.Items[0].Assignee;
+        for (var i = 1; i < 10; i++)
+        {
+            Assert.AreSame(firstAssignee, sameIIdResult.Items[i].Assignee,
+                $"CRITICAL: Items[{i}].Assignee should be same reference as Items[0].Assignee (same IId.Id=1)");
+        }
+    }
+
+    #endregion
+
+    #region Guid-Based IId Tests
+
+    /// <summary>
+    /// Guid IId with same instance - validates $ref + data integrity + reference identity.
+    /// </summary>
+    [TestMethod]
+    public void GuidIId_SameInstance_SerializeAndDeserialize()
+    {
+        // Arrange: SAME instance
+        var sharedGuid = Guid.NewGuid();
+        var sharedItem = new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" };
+        
+        var order = new TestGuidOrder
+        {
+            Id = Guid.NewGuid(),
+            Code = "GUID-001",
+            Count = 3,
+            Items = [sharedItem, sharedItem, sharedItem]
+        };
+
+        // Act
+        var json = order.ToJson();
+        Console.WriteLine(json);
+        var result = json.JsonTo<TestGuidOrder>();
+
+        // Assert 1: $ref should be present
+        var refCount = CountOccurrences(json, "{\"$ref\":\"1\"}");
+        Console.WriteLine($"JSON length: {json.Length} chars");
+        Console.WriteLine($"$ref count: {refCount}");
+        
+        Assert.IsTrue(refCount >= 2, $"Expected at least 2 $refs, found {refCount}");
+
+        // Assert 2: Data integrity - all items present and correct
+        Assert.IsNotNull(result, "Result is null");
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            Assert.IsNotNull(result.Items[i], $"Items[{i}] is null");
+            Assert.AreEqual(sharedGuid, result.Items[i].Id, $"Items[{i}].Id incorrect");
+            Assert.AreEqual("SharedGuidItem", result.Items[i].Name, $"Items[{i}].Name incorrect");
+        }
+
+        // Assert 3: Reference identity
+        Assert.AreSame(result.Items[0], result.Items[1], "Items[0] and Items[1] should be same reference");
+        Assert.AreSame(result.Items[1], result.Items[2], "Items[1] and Items[2] should be same reference");
+    }
+
+    /// <summary>
+    /// CRITICAL: Guid IId with different instances but same Id - tests IId-based deduplication.
+    /// </summary>
+    [TestMethod]
+    public void GuidIId_DifferentInstances_SameId_SerializeAndDeserialize()
+    {
+        // Arrange: DIFFERENT instances but SAME Guid
+        var sharedGuid = Guid.NewGuid();
+        
+        var order = new TestGuidOrder
+        {
+            Id = Guid.NewGuid(),
+            Code = "GUID-001",
+            Count = 3,
+            Items =
+            [
+                new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" },
+                new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" },
+                new TestGuidItem { Id = sharedGuid, Name = "SharedGuidItem" }
+            ]
+        };
+
+        // Act
+        var json = order.ToJson();
+        var result = json.JsonTo<TestGuidOrder>();
+
+        // Assert 1: $ref should be present if IId-based dedup works
+        var refCount = CountOccurrences(json, "\"$ref\"");
+        Console.WriteLine($"JSON length: {json.Length} chars");
+        Console.WriteLine($"$ref count: {refCount}");
+        Console.WriteLine("JSON:");
+        Console.WriteLine(json);
+        
+        Assert.IsTrue(refCount >= 2, 
+            $"CRITICAL: Expected at least 2 $refs for same Guid IId, found {refCount}. " +
+            "Guid-based IId deduplication NOT working!");
+
+        // Assert 2: Data integrity - all items present and correct
+        Assert.IsNotNull(result, "Result is null");
+        Assert.IsNotNull(result.Items, "Items is null");
+        Assert.AreEqual(3, result.Items.Count, "Items count incorrect");
+        
+        for (var i = 0; i < 3; i++)
+        {
+            Assert.IsNotNull(result.Items[i], $"Items[{i}] is null - data lost!");
+            Assert.AreEqual(sharedGuid, result.Items[i].Id, $"Items[{i}].Id incorrect");
+            Assert.AreEqual("SharedGuidItem", result.Items[i].Name, $"Items[{i}].Name incorrect");
+        }
+
+        // Assert 3: Reference identity - if IId-based, should be same reference
+        Assert.AreSame(result.Items[0], result.Items[1], 
+            "CRITICAL: Items[0] and Items[1] should be same reference (same Guid)");
+        Assert.AreSame(result.Items[1], result.Items[2], 
+            "CRITICAL: Items[1] and Items[2] should be same reference (same Guid)");
+    }
+
+    #endregion
+
+    #region Data Integrity Tests
+
+    [TestMethod]
+    public void SharedCategory_DataIntegrity()
+    {
+        var categories = new List<SharedCategory>
+        {
+            new() { Id = 1, Name = "Category1", SortOrder = 1, IsDefault = true },
+            new() { Id = 2, Name = "Category2", SortOrder = 2, ParentCategoryId = 1 },
+            new() { Id = 3, Name = "Category3", SortOrder = 3, ParentCategoryId = 1 }
+        };
+
+        var json = categories.ToJson();
+        var result = json.JsonTo<List<SharedCategory>>();
+
+        Assert.IsNotNull(result);
+        Assert.AreEqual(3, result.Count);
+        
+        Assert.AreEqual(1, result[0].Id);
+        Assert.AreEqual("Category1", result[0].Name);
+        Assert.IsTrue(result[0].IsDefault);
+        
+        Assert.AreEqual(2, result[1].Id);
+        Assert.AreEqual(1, result[1].ParentCategoryId);
+    }
+
+    #endregion
+}

AyCode.Core/Serializers/Toons/AcToonSerializer.MetaWriter.cs

@@ -9,15 +9,15 @@ public static partial class AcToonSerializer
     /// <summary>
     /// Write meta section only (for MetaOnly mode).
     /// </summary>
-    private static void WriteMetaSectionOnly(Type type, ToonSerializationContext context)
+    private static void WriteMetaSectionOnly(Type type, ToonSerializationContext context, string domainContext)
     {
-        WriteMetaSection(type, context);
+        WriteMetaSection(type, context, domainContext);
     }
 
     /// <summary>
     /// Write @meta and @types sections for a single root type.
     /// </summary>
-    private static void WriteMetaSection(Type type, ToonSerializationContext context)
+    private static void WriteMetaSection(Type type, ToonSerializationContext context, string domainContext)
     {
         if (!context.Options.UseMeta) return;
 
@@ -25,13 +25,13 @@ public static partial class AcToonSerializer
         var typesToDocument = new HashSet<Type>();
         CollectTypes(type, typesToDocument);
 
-        WriteMetaSectionCore(typesToDocument, context);
+        WriteMetaSectionCore(typesToDocument, context, domainContext);
     }
 
     /// <summary>
     /// Write @meta and @types sections for a collection of types.
     /// </summary>
-    private static void WriteMetaSection(IEnumerable<Type> types, ToonSerializationContext context)
+    private static void WriteMetaSection(IEnumerable<Type> types, ToonSerializationContext context, string domainContext)
     {
         if (!context.Options.UseMeta) return;
 
@@ -42,13 +42,13 @@ public static partial class AcToonSerializer
             CollectTypes(type, typesToDocument);
         }
 
-        WriteMetaSectionCore(typesToDocument, context);
+        WriteMetaSectionCore(typesToDocument, context, domainContext);
     }
 
     /// <summary>
     /// Core logic for writing @meta and @types sections.
     /// </summary>
-    private static void WriteMetaSectionCore(HashSet<Type> typesToDocument, ToonSerializationContext context)
+    private static void WriteMetaSectionCore(HashSet<Type> typesToDocument, ToonSerializationContext context, string domainContext)
     {
         // @meta header
         context.WriteLine("@meta {");
@@ -57,6 +57,9 @@ public static partial class AcToonSerializer
         context.WriteProperty("format", "\"toon\"");
         context.WriteProperty("source-code-language", "\"C#\"");
 
+        if(!string.IsNullOrEmpty(domainContext))
+            context.WriteProperty("context", $"\"{domainContext}\"");
+
         // Write type list
         context.WriteIndent();
         context.Write("types");

AyCode.Core/Serializers/Toons/AcToonSerializer.cs

@@ -34,7 +34,8 @@ public static partial class AcToonSerializer
     /// <summary>
     /// Serialize object to Toon format with specified options.
     /// </summary>
-    public static string Serialize<T>(T value, AcToonSerializerOptions options)
+    public static string Serialize<T>(T value, AcToonSerializerOptions options) => Serialize<T>(value, string.Empty, options);
+    public static string Serialize<T>(T value, string domainDescription, AcToonSerializerOptions options)
     {
         if (value == null) return "null";
 
@@ -57,7 +58,7 @@ public static partial class AcToonSerializer
             switch (options.Mode)
             {
                 case ToonSerializationMode.MetaOnly:
-                    WriteMetaSectionOnly(type, context);
+                    WriteMetaSectionOnly(type, context, domainDescription);
                     break;
 
                 case ToonSerializationMode.DataOnly:
@@ -66,7 +67,7 @@ public static partial class AcToonSerializer
 
                 case ToonSerializationMode.Full:
                 default:
-                    WriteMetaSection(type, context);
+                    WriteMetaSection(type, context, domainDescription);
                     context.WriteLine();
                     WriteDataSection(value, type, context);
                     break;
@@ -84,17 +85,19 @@ public static partial class AcToonSerializer
     /// Serialize only type metadata (schema) for a given type.
     /// Useful for sending type information once at conversation start.
     /// </summary>
-    public static string SerializeTypeMetadata<T>() => SerializeTypeMetadata(typeof(T));
+    public static string SerializeTypeMetadata<T>() => SerializeTypeMetadata(typeof(T), string.Empty);
+    public static string SerializeTypeMetadata<T>(string domainDescription) => SerializeTypeMetadata(typeof(T), domainDescription);
 
     /// <summary>
     /// Serialize only type metadata (schema) for a given type.
     /// </summary>
-    public static string SerializeTypeMetadata(Type type)
+    public static string SerializeTypeMetadata(Type type) => SerializeTypeMetadata(type, string.Empty);
+    public static string SerializeTypeMetadata(Type type, string domainDescription)
     {
         var context = ToonSerializationContextPool.Get(AcToonSerializerOptions.MetaOnly);
         try
         {
-            WriteMetaSectionOnly(type, context);
+            WriteMetaSectionOnly(type, context, domainDescription);
             return context.GetResult();
         }
         finally
@@ -110,7 +113,8 @@ public static partial class AcToonSerializer
     /// <param name="types">Types to document</param>
     /// <param name="options">Serialization options (optional, defaults to MetaOnly preset)</param>
     /// <returns>Metadata-only Toon format string with @meta and @types sections</returns>
-    public static string SerializeMetadata(IEnumerable<Type> types, AcToonSerializerOptions? options = null)
+    public static string SerializeMetadata(IEnumerable<Type> types, AcToonSerializerOptions? options = null) => SerializeMetadata(types, string.Empty, options);
+    public static string SerializeMetadata(IEnumerable<Type> types, string domainDescription, AcToonSerializerOptions? options = null)
     {
         // Return empty string if no types provided
         var typesList = types?.ToList();
@@ -124,7 +128,7 @@ public static partial class AcToonSerializer
         var context = ToonSerializationContextPool.Get(options);
         try
         {
-            WriteMetaSection(typesList, context);
+            WriteMetaSection(typesList, context, domainDescription);
             return context.GetResult();
         }
         finally
@@ -138,11 +142,8 @@ public static partial class AcToonSerializer
     /// </summary>
     /// <param name="types">Types to document</param>
     /// <returns>Metadata-only Toon format string with @meta and @types sections</returns>
-    public static string SerializeMetadata(params Type[] types)
+    public static string SerializeMetadata(params Type[] types) => SerializeMetadata((IEnumerable<Type>)types);
-    {
+    public static string SerializeMetadata(string domainDescription, params Type[] types) => SerializeMetadata((IEnumerable<Type>)types, domainDescription);
-        return SerializeMetadata((IEnumerable<Type>)types);
-    }
-
     #endregion
 
     #region Primitive Serialization