Add unit tests for shared reference instance serialization

tests/SerializationModels.cs

@@ -76,7 +76,7 @@
 }
 
 [ZeroSerializer]
 public readonly struct EnumStruct
 {
     public EnumStruct(ByteState byteState, SignedState signedState)
     {
@@ -149,7 +149,7 @@
 }
 
 [ZeroSerializer]
 public readonly struct SmallFixedStruct
 {
     public SmallFixedStruct(int value)
     {
@@ -160,7 +160,7 @@
 }
 
 [ZeroSerializer]
 public readonly struct LargeFixedStruct
 {
     public LargeFixedStruct(long value, ByteState state)
     {
@@ -294,6 +294,27 @@
     public int Value { get; init; }
 }
 
+[ZeroSerializer]
+public class DuplicateInstanceContainer
+{
+    public SharedClassItem? Foo { get; set; }
+    public SharedClassItem? Bar { get; set; }
+    public SharedClassItem? Baz { get; set; }
+}
+
+[ZeroSerializer]
+public class SharedClassItem
+{
+    public int Value { get; set; }
+    public SharedClassNested? Nested { get; set; }
+}
+
+[ZeroSerializer]
+public class SharedClassNested
+{
+    public int NestedValue { get; set; }
+}
+
 [ZeroSerializer]
 public record SimpleCsharpRecord
 {

tests/SerializationTests.cs

@@ -1228,4 +1228,34 @@ public void BlittableRecordStructNestedPropertyRoundTrip()
         TestAssert.True(viewNulls.Values.IsEmpty, nameof(viewNulls.Values.IsEmpty));
         TestAssert.Equal(writtenBytesNulls, viewNulls.GetByteLength(), "Nulls GetByteLength");
     }
+
+    [Fact]
+    public void SharedReferenceInstancesRoundTrip()
+    {
+        var sharedNested = new SharedClassNested { NestedValue = 42 };
+        var sharedItem = new SharedClassItem { Value = 100, Nested = sharedNested };
+        var distinctItemWithSharedNested = new SharedClassItem { Value = 200, Nested = sharedNested };
+
+        // Foo and Bar share SharedClassItem instance; Baz has a distinct SharedClassItem instance but shares the same SharedClassNested instance
+        var container = new DuplicateInstanceContainer
+        {
+            Foo = sharedItem,
+            Bar = sharedItem,
+            Baz = distinctItemWithSharedNested,
+        };
+
+        var buffer = new byte[256];
+        int writtenBytes = container.Serialize(buffer);
+        var view = new DuplicateInstanceContainerView(buffer.AsMemory(0, writtenBytes));
+
+        TestAssert.Equal(100, view.Foo.Value, nameof(view.Foo.Value));
+        TestAssert.Equal(100, view.Bar.Value, nameof(view.Bar.Value));
+        TestAssert.Equal(200, view.Baz.Value, nameof(view.Baz.Value));
+
+        TestAssert.Equal(42, view.Foo.Nested.NestedValue, nameof(view.Foo.Nested.NestedValue));
+        TestAssert.Equal(42, view.Bar.Nested.NestedValue, nameof(view.Bar.Nested.NestedValue));
+        TestAssert.Equal(42, view.Baz.Nested.NestedValue, nameof(view.Baz.Nested.NestedValue));
+
+        TestAssert.Equal(writtenBytes, view.GetByteLength(), "SharedReferenceInstances GetByteLength");
+    }
 }