Fix AIFunctionFactory handling of default struct arguments (#6381)

* Fix AIFunctionFactory handling of default struct arguments

* Extend testing to custom structs

* Use GetUninitializedObject instead of Activator

Author: eiriktsarpalis

src/Libraries/Microsoft.Extensions.AI.Abstractions/Utilities/AIJsonUtilities.Schema.cs

@@ -446,21 +446,23 @@ private static JsonElement ParseJsonElement(ReadOnlySpan<byte> utf8Json)
         return JsonElement.ParseValue(ref reader);
     }
 
+    [UnconditionalSuppressMessage("Trimming", "IL2072:Target parameter argument does not satisfy 'DynamicallyAccessedMembersAttribute' in call to target method.",
+        Justification = "Called conditionally on structs whose default ctor never gets trimmed.")]
     private static object? GetDefaultValueNormalized(ParameterInfo parameterInfo)
     {
         // Taken from https://github.com/dotnet/runtime/blob/eff415bfd667125c1565680615a6f19152645fbf/src/libraries/System.Text.Json/Common/ReflectionExtensions.cs#L288-L317
         Type parameterType = parameterInfo.ParameterType;
         object? defaultValue = parameterInfo.DefaultValue;
 
-        if (defaultValue is null)
+        if (defaultValue is null || (defaultValue == DBNull.Value && parameterType != typeof(DBNull)))
         {
-            return null;
-        }
-
-        // DBNull.Value is sometimes used as the default value (returned by reflection) of nullable params in place of null.
-        if (defaultValue == DBNull.Value && parameterType != typeof(DBNull))
-        {
-            return null;
+            return parameterType.IsValueType
+#if NET
+                ? RuntimeHelpers.GetUninitializedObject(parameterType)
+#else
+                ? System.Runtime.Serialization.FormatterServices.GetUninitializedObject(parameterType)
+#endif
+                : null;
         }
 
         // Default values of enums or nullable enums are represented using the underlying type and need to be cast explicitly

test/Libraries/Microsoft.Extensions.AI.Tests/Functions/AIFunctionFactoryTest.cs

@@ -809,6 +809,17 @@ public async Task MarshalResult_TypeIsDeclaredTypeEvenWhenDerivedTypeReturned()
         Assert.Equal("marshalResultInvoked", result);
     }
 
+    [Fact]
+    public async Task AIFunctionFactory_DefaultDefaultParameter()
+    {
+        Assert.NotEqual(new StructWithDefaultCtor().Value, default(StructWithDefaultCtor).Value);
+
+        AIFunction f = AIFunctionFactory.Create((Guid g = default, StructWithDefaultCtor s = default) => g.ToString() + "," + s.Value.ToString(), serializerOptions: JsonContext.Default.Options);
+
+        object? result = await f.InvokeAsync();
+        Assert.Contains("00000000-0000-0000-0000-000000000000,0", result?.ToString());
+    }
+
     private sealed class MyService(int value)
     {
         public int Value => value;
@@ -871,7 +882,19 @@ private class A;
     private class B : A;
     private sealed class C : B;
 
+    public readonly struct StructWithDefaultCtor
+    {
+        public int Value { get; }
+        public StructWithDefaultCtor()
+        {
+            Value = 42;
+        }
+    }
+
     [JsonSerializable(typeof(IAsyncEnumerable<int>))]
     [JsonSerializable(typeof(int[]))]
+    [JsonSerializable(typeof(string))]
+    [JsonSerializable(typeof(Guid))]
+    [JsonSerializable(typeof(StructWithDefaultCtor))]
     private partial class JsonContext : JsonSerializerContext;
 }