Support Polymorphic input payload deserialization (isolated dotnet)

release_notes.md

@@ -20,6 +20,8 @@
 
 ### Bug Fixes
 
+- Support Polymorphic input payload deserialization (https://github.com/Azure/azure-functions-durable-extension/pull/3250)
+
 ### Breaking Changes
 
 ### Dependency Updates

src/Worker.Extensions.DurableTask/ObjectConverterShim.cs

@@ -3,7 +3,9 @@
 
 using System;
 using System.IO;
+using System.Reflection;
 using System.Text;
+using System.Text.Json.Serialization;
 using Azure.Core.Serialization;
 using Microsoft.DurableTask;
 
@@ -39,7 +41,69 @@ public ObjectConverterShim(ObjectSerializer serializer)
             return null;
         }
 
-        BinaryData data = this.serializer.Serialize(value, value.GetType(), default);
+        Type valueType = value.GetType();
+
+        // Special handling for object[] arrays - DTFx wraps activity inputs in object[]
+        // When System.Text.Json serializes object[], it treats elements as type 'object' (the static array element type),
+        // not as their runtime concrete type, which prevents polymorphic type discriminators from being added.
+        // We must serialize each element individually with its polymorphic base type and build the JSON array manually.
+        if (valueType == typeof(object[]))
+        {
+            object[] array = (object[])value;
+            var jsonBuilder = new StringBuilder();
+            jsonBuilder.Append('[');
+
+            for (int i = 0; i < array.Length; i++)
+            {
+                if (i > 0)
+                {
+                    jsonBuilder.Append(',');
+                }
+
+                object? element = array[i];
+                if (element != null)
+                {
+                    // Serialize each element with its polymorphic base type to include type discriminators
+                    Type elementType = element.GetType();
+                    Type serializeAs = GetPolymorphicBaseType(elementType) ?? elementType;
+
+                    BinaryData elementData = this.serializer.Serialize(element, serializeAs, default);
+                    jsonBuilder.Append(elementData.ToString());
+                }
+                else
+                {
+                    jsonBuilder.Append("null");
+                }
+            }
+
+            jsonBuilder.Append(']');
+            return jsonBuilder.ToString();
+        }
+
+        // For non-array values (or non-object[] arrays), serialize as polymorphic base type
+        Type typeToSerialize = GetPolymorphicBaseType(valueType) ?? valueType;
+
+        BinaryData data = this.serializer.Serialize(value, typeToSerialize, default);
         return data.ToString();
     }
+
+    /// <summary>
+    /// Finds the polymorphic base type for a given type by walking up the inheritance chain.
+    /// Returns the first base type decorated with <see cref="JsonPolymorphicAttribute"/>.
+    /// </summary>
+    /// <param name="concreteType">The concrete type to check.</param>
+    /// <returns>The polymorphic base type if found, otherwise null.</returns>
+    private static Type? GetPolymorphicBaseType(Type concreteType)
+    {
+        Type? current = concreteType.BaseType;
+        while (current != null && current != typeof(object))
+        {
+            if (current.GetCustomAttribute<JsonPolymorphicAttribute>() != null)
+            {
+                return current;
+            }
+            current = current.BaseType;
+        }
+        return null;
+    }
 }