Add support for serializing properties in interface hierarchies

src/libraries/System.Text.Json/Common/ReflectionExtensions.cs

@@ -324,5 +324,111 @@ public static bool TryGetDeserializationConstructor(
 
             return defaultValue;
         }
+
+        /// <summary>
+        /// Returns the type hierarchy for the given type, starting from the current type up to the base type(s) in the hierarchy.
+        /// Interface hierarchies with multiple inheritance will return results using topological sorting.
+        /// </summary>
+        public static Type[] GetSortedTypeHierarchy(
+#if !BUILDING_SOURCE_GENERATOR
+            [DynamicallyAccessedMembers(DynamicallyAccessedMemberTypes.Interfaces)]
+#endif
+            this Type type)
+        {
+            if (!type.IsInterface)
+            {
+                // Non-interface hierarchies are linear, just walk up to the earliest ancestor.
+
+                var results = new List<Type>();
+                for (Type? current = type; current != null; current = current.BaseType)
+                {
+                    results.Add(current);
+                }
+
+                return results.ToArray();
+            }
+            else
+            {
+                // Interface hierarchies support multiple inheritance,
+                // query the entire list and sort them topologically.
+                Type[] interfaces = type.GetInterfaces();
+                {
+                    // include the current type into the list of interfaces
+                    Type[] newArray = new Type[interfaces.Length + 1];
+                    newArray[0] = type;
+                    interfaces.CopyTo(newArray, 1);
+                    interfaces = newArray;
+                }
+
+                TopologicalSort(interfaces, static (t1, t2) => t1.IsAssignableFrom(t2));
+                return interfaces;
+            }
+        }
+
+        private static void TopologicalSort<T>(T[] inputs, Func<T, T, bool> isLessThan)
+            where T : notnull
+        {
+            // Standard implementation of in-place topological sorting using Kahn's algorithm.
+
+            if (inputs.Length < 2)
+            {
+                return;
+            }
+
+            var graph = new Dictionary<T, HashSet<T>>();
+            var next = new Queue<T>();
+
+            // Step 1: construct the dependency graph.
+            for (int i = 0; i < inputs.Length; i++)
+            {
+                T current = inputs[i];
+                HashSet<T>? dependencies = null;
+
+                for (int j = 0; j < inputs.Length; j++)
+                {
+                    if (i != j && isLessThan(current, inputs[j]))
+                    {
+                        (dependencies ??= new()).Add(inputs[j]);
+                    }
+                }
+
+                if (dependencies is null)
+                {
+                    next.Enqueue(current);
+                }
+                else
+                {
+                    graph.Add(current, dependencies);
+                }
+            }
+
+            Debug.Assert(next.Count > 0, "Input graph must be a DAG.");
+            int index = 0;
+
+            // Step 2: Walk the dependency graph starting with nodes that have no dependencies.
+            do
+            {
+                T nextTopLevelDependency = next.Dequeue();
+
+                foreach (KeyValuePair<T, HashSet<T>> kvp in graph)
+                {
+                    HashSet<T> dependencies = kvp.Value;
+                    if (dependencies.Count > 0)
+                    {
+                        dependencies.Remove(nextTopLevelDependency);
+
+                        if (dependencies.Count == 0)
+                        {
+                            next.Enqueue(kvp.Key);
+                        }
+                    }
+                }
+
+                inputs[index++] = nextTopLevelDependency;
+            }
+            while (next.Count > 0);
+
+            Debug.Assert(index == inputs.Length);
+        }
     }
 }

src/libraries/System.Text.Json/gen/JsonSourceGenerator.Parser.cs

@@ -1026,7 +1026,8 @@ private TypeGenerationSpec GetOrAddTypeGenerationSpec(Type type, JsonSourceGener
 
                         bool propertyOrderSpecified = false;
 
-                        for (Type? currentType = type; currentType != null; currentType = currentType.BaseType)
+                        // Walk the type hierarchy starting from the current type up to the base type(s)
+                        foreach (Type currentType in type.GetSortedTypeHierarchy())
                         {
                             PropertyGenerationSpec spec;
 
@@ -1260,6 +1261,7 @@ private PropertyGenerationSpec GetPropertyGenerationSpec(
                     IsExtensionData = isExtensionData,
                     TypeGenerationSpec = GetOrAddTypeGenerationSpec(memberCLRType, generationMode),
                     DeclaringTypeRef = memberInfo.DeclaringType.GetCompilableName(),
+                    DeclaringType = memberInfo.DeclaringType,
                     ConverterInstantiationLogic = converterInstantiationLogic,
                     HasFactoryConverter = hasFactoryConverter
                 };

src/libraries/System.Text.Json/gen/PropertyGenerationSpec.cs

@@ -110,6 +110,8 @@ internal sealed class PropertyGenerationSpec
         /// </summary>
         public string DeclaringTypeRef { get; init; }
 
+        public Type DeclaringType { get; init; }
+
         /// <summary>
         /// Source code to instantiate design-time specified custom converter.
         /// </summary>

src/libraries/System.Text.Json/gen/TypeGenerationSpec.cs

@@ -166,6 +166,7 @@ public bool TryFilterSerializableProps(
                 [NotNullWhen(true)] out Dictionary<string, PropertyGenerationSpec>? serializableProperties,
                 out bool castingRequiredForProps)
         {
+            castingRequiredForProps = false;
             serializableProperties = new Dictionary<string, PropertyGenerationSpec>();
             Dictionary<string, PropertyGenerationSpec>? ignoredMembers = null;
 
@@ -198,6 +199,10 @@ public bool TryFilterSerializableProps(
                     continue;
                 }
 
+                // Using properties from an interface hierarchy -- require explicit casting when
+                // getting properties in the fast path to account for possible diamond ambiguities.
+                castingRequiredForProps |= Type.IsInterface && propGenSpec.DeclaringType != Type;
+
                 string memberName = propGenSpec.ClrName!;
 
                 // The JsonPropertyNameAttribute or naming policy resulted in a collision.
@@ -210,32 +215,52 @@ public bool TryFilterSerializableProps(
                         // Overwrite previously cached property since it has [JsonIgnore].
                         serializableProperties[propGenSpec.RuntimePropertyName] = propGenSpec;
                     }
-                    else if (
-                        // Does the current property have `JsonIgnoreAttribute`?
-                        propGenSpec.DefaultIgnoreCondition != JsonIgnoreCondition.Always &&
-                        // Is the current property hidden by the previously cached property
-                        // (with `new` keyword, or by overriding)?
-                        other.ClrName != memberName &&
-                        // Was a property with the same CLR name was ignored? That property hid the current property,
-                        // thus, if it was ignored, the current property should be ignored too.
-                        ignoredMembers?.ContainsKey(memberName) != true)
+                    else
                     {
-                        // We throw if we have two public properties that have the same JSON property name, and neither have been ignored.
-                        serializableProperties = null;
-                        castingRequiredForProps = false;
-                        return false;
+                        bool ignoreCurrentProperty;
+
+                        if (!Type.IsInterface)
+                        {
+                            ignoreCurrentProperty =
+                                // Does the current property have `JsonIgnoreAttribute`?
+                                propGenSpec.DefaultIgnoreCondition == JsonIgnoreCondition.Always ||
+                                // Is the current property hidden by the previously cached property
+                                // (with `new` keyword, or by overriding)?
+                                other.ClrName == memberName ||
+                                // Was a property with the same CLR name ignored? That property hid the current property,
+                                // thus, if it was ignored, the current property should be ignored too.
+                                ignoredMembers?.ContainsKey(memberName) == true;
+                        }
+                        else
+                        {
+                            // Unlike classes, interface hierarchies reject all naming conflicts for non-ignored properties.
+                            // Conflicts like this are possible in two cases:
+                            // 1. Diamond ambiguity in property names, or
+                            // 2. Linear interface hierarchies that use properties with DIMs.
+                            //
+                            // Diamond ambiguities are not supported. Assuming there is demand, we might consider
+                            // adding support for DIMs in the future, however that would require adding more APIs
+                            // for the case of source gen.
+
+                            ignoreCurrentProperty = propGenSpec.DefaultIgnoreCondition == JsonIgnoreCondition.Always;
+                        }
+
+                        if (!ignoreCurrentProperty)
+                        {
+                            // We have a conflict, emit a stub method that throws.
+                            goto ReturnFalse;
+                        }
                     }
-                    // Ignore the current property.
                 }
 
                 if (propGenSpec.DefaultIgnoreCondition == JsonIgnoreCondition.Always)
                 {
-                    (ignoredMembers ??= new Dictionary<string, PropertyGenerationSpec>()).Add(memberName, propGenSpec);
+                    (ignoredMembers ??= new()).Add(memberName, propGenSpec);
                 }
             }
 
             Debug.Assert(PropertyGenSpecList.Count >= serializableProperties.Count);
-            castingRequiredForProps = PropertyGenSpecList.Count > serializableProperties.Count;
+            castingRequiredForProps |= PropertyGenSpecList.Count > serializableProperties.Count;
             return true;
 
         ReturnFalse:

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

@@ -791,25 +791,46 @@ internal void CacheMember(JsonPropertyInfo jsonPropertyInfo, JsonPropertyDiction
                     // Overwrite previously cached property since it has [JsonIgnore].
                     propertyCache[jsonPropertyInfo.Name] = jsonPropertyInfo;
                 }
-                else if (
-                    // Does the current property have `JsonIgnoreAttribute`?
-                    !jsonPropertyInfo.IsIgnored &&
-                    // Is the current property hidden by the previously cached property
-                    // (with `new` keyword, or by overriding)?
-                    other.MemberName != memberName &&
-                    // Was a property with the same CLR name was ignored? That property hid the current property,
-                    // thus, if it was ignored, the current property should be ignored too.
-                    ignoredMembers?.ContainsKey(memberName) != true)
+                else
                 {
-                    // We throw if we have two public properties that have the same JSON property name, and neither have been ignored.
-                    ThrowHelper.ThrowInvalidOperationException_SerializerPropertyNameConflict(Type, jsonPropertyInfo.Name);
+                    bool ignoreCurrentProperty;
+
+                    if (!Type.IsInterface)
+                    {
+                        ignoreCurrentProperty =
+                            // Does the current property have `JsonIgnoreAttribute`?
+                            jsonPropertyInfo.IsIgnored ||
+                            // Is the current property hidden by the previously cached property
+                            // (with `new` keyword, or by overriding)?
+                            other.MemberName == memberName ||
+                            // Was a property with the same CLR name ignored? That property hid the current property,
+                            // thus, if it was ignored, the current property should be ignored too.
+                            ignoredMembers?.ContainsKey(memberName) == true;
+                    }
+                    else
+                    {
+                        // Unlike classes, interface hierarchies reject all naming conflicts for non-ignored properties.
+                        // Conflicts like this are possible in two cases:
+                        // 1. Diamond ambiguity in property names, or
+                        // 2. Linear interface hierarchies that use properties with DIMs.
+                        //
+                        // Diamond ambiguities are not supported. Assuming there is demand, we might consider
+                        // adding support for DIMs in the future, however that would require adding more APIs
+                        // for the case of source gen.
+
+                        ignoreCurrentProperty = jsonPropertyInfo.IsIgnored;
+                    }
+
+                    if (!ignoreCurrentProperty)
+                    {
+                        ThrowHelper.ThrowInvalidOperationException_SerializerPropertyNameConflict(Type, jsonPropertyInfo.Name);
+                    }
                 }
-                // Ignore the current property.
             }
 
             if (jsonPropertyInfo.IsIgnored)
             {
-                (ignoredMembers ??= new Dictionary<string, JsonPropertyInfo>()).Add(memberName, jsonPropertyInfo);
+                (ignoredMembers ??= new()).Add(memberName, jsonPropertyInfo);
             }
         }