[STJ] Account for F# CompilationMappingAttribute now supporting multiple declarations.

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Metadata/FSharpCoreReflectionProxy.cs

@@ -204,15 +204,18 @@ public Func<IEnumerable<Tuple<TKey, TValue>>, TFSharpMap> CreateFSharpMapConstru
         }
 
         private Attribute? GetFSharpCompilationMappingAttribute(Type type)
-            => type.GetCustomAttribute(_compilationMappingAttributeType, inherit: true);
+        {
+            object[] attributes = type.GetCustomAttributes(_compilationMappingAttributeType, inherit: false);
+            return attributes.Length == 0 ? null : (Attribute)attributes[0];
+        }
 
         private SourceConstructFlags GetSourceConstructFlags(Attribute compilationMappingAttribute)
             => _sourceConstructFlagsGetter is null ? SourceConstructFlags.None : (SourceConstructFlags)_sourceConstructFlagsGetter.Invoke(compilationMappingAttribute, null)!;
 
         // If the provided type is generated by the F# compiler, returns the runtime FSharp.Core assembly.
         private static Assembly? GetFSharpCoreAssembly(Type type)
         {
-            foreach (Attribute attr in type.GetCustomAttributes(inherit: true))
+            foreach (Attribute attr in type.GetCustomAttributes(inherit: false))
             {
                 Type attributeType = attr.GetType();
                 if (attributeType.FullName == CompilationMappingAttributeTypeName)

src/libraries/System.Text.Json/tests/System.Text.Json.FSharp.Tests/RecordTests.fs

@@ -76,3 +76,22 @@ let ``Recursive struct records are supported``() =
     Assert.Equal("""{"Next":[{"Next":[null]}]}""", json)
     let deserializedValue = JsonSerializer.Deserialize<RecursiveStructRecord>(json)
     Assert.Equal(value, deserializedValue)
+
+
+[<JsonDerivedType(typeof<DerivedClass>, "derived")>]
+type BaseClass(x : int) =
+    member _.X = x
+
+and DerivedClass(x : int, y : bool) = 
+    inherit BaseClass(x)
+    member _.Y = y
+
+[<Fact>]
+let ``Support F# class hierarchies`` () =
+    let value = DerivedClass(42, true) :> BaseClass
+    let json = JsonSerializer.Serialize(value)
+    Assert.Equal("""{"$type":"derived","Y":true,"X":42}""", json)
+    let deserializedValue = JsonSerializer.Deserialize<BaseClass>(json)
+    let derived = Assert.IsType<DerivedClass>(deserializedValue)
+    Assert.Equal(42, deserializedValue.X)
+    Assert.Equal(true, derived.Y)