Support DefaultValueAttribute in AIFunctionFactory parameter handling (#6947)

* Initial plan

* Add support for DefaultValue attribute in AIFunctionFactory

- Check for DefaultValueAttribute when determining parameter optionality
- Use DefaultValueAttribute value as default when parameter not provided
- Add helper methods HasEffectiveDefaultValue and GetEffectiveDefaultValue
- Add comprehensive tests for DefaultValue attribute support

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Add test for DefaultValue attribute precedence over C# default

Verify that when both DefaultValue attribute and C# default are present,
the DefaultValue attribute takes precedence.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Add test for DefaultValue precedence and update gitignore

- Add test verifying DefaultValue attribute takes precedence over C# default
- Add .nuget directory to gitignore to exclude build artifacts

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Address PR feedback: consolidate helpers and fix formatting

- Revert changes to .gitignore
- Fix formatting issues in test file (remove extra blank lines)
- Make HasEffectiveDefaultValue and GetEffectiveDefaultValue internal in AIJsonUtilities
- Remove duplicate helper methods from AIFunctionFactory and use the ones from AIJsonUtilities

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Rename parameter 's' to 'text' in test to avoid false positives

Use a longer, more descriptive parameter name to avoid false positives
when searching for the parameter name in string assertions.

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

* Unify helper methods into TryGetEffectiveDefaultValue

Replace HasEffectiveDefaultValue and GetEffectiveDefaultValue with a single
TryGetEffectiveDefaultValue method to avoid resolving the DefaultValueAttribute
multiple times, improving performance.

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: 3 people

.nuget/nuget.exe

@@ -844,10 +844,11 @@ static bool IsAsyncMethod(MethodInfo method)
             // For IServiceProvider parameters, we bind to the services passed to InvokeAsync via AIFunctionArguments.
             if (parameterType == typeof(IServiceProvider))
             {
+                bool hasDefault = AIJsonUtilities.TryGetEffectiveDefaultValue(parameter, out _);
                 return (arguments, _) =>
                 {
                     IServiceProvider? services = arguments.Services;
-                    if (!parameter.HasDefaultValue && services is null)
+                    if (!hasDefault && services is null)
                     {
                         ThrowNullServices(parameter.Name);
                     }
@@ -859,6 +860,7 @@ static bool IsAsyncMethod(MethodInfo method)
             // For all other parameters, create a marshaller that tries to extract the value from the arguments dictionary.
             // Resolve the contract used to marshal the value from JSON -- can throw if not supported or not found.
             JsonTypeInfo? typeInfo = serializerOptions.GetTypeInfo(parameterType);
+            bool hasDefaultValue = AIJsonUtilities.TryGetEffectiveDefaultValue(parameter, out object? effectiveDefaultValue);
             return (arguments, _) =>
             {
                 // If the parameter has an argument specified in the dictionary, return that argument.
@@ -907,13 +909,13 @@ static bool IsAsyncMethod(MethodInfo method)
                 }
 
                 // If the parameter is required and there's no argument specified for it, throw.
-                if (!parameter.HasDefaultValue)
+                if (!hasDefaultValue)
                 {
                     Throw.ArgumentException(nameof(arguments), $"The arguments dictionary is missing a value for the required parameter '{parameter.Name}'.");
                 }
 
                 // Otherwise, use the optional parameter's default value.
-                return parameter.DefaultValue;
+                return effectiveDefaultValue;
             };
 
             // Throws an ArgumentNullException indicating that AIFunctionArguments.Services must be provided.

src/Libraries/Microsoft.Extensions.AI.Abstractions/Functions/AIFunctionFactory.cs

@@ -108,17 +108,18 @@ public static JsonElement CreateFunctionJsonSchema(
                 continue;
             }
 
+            bool hasDefaultValue = TryGetEffectiveDefaultValue(parameter, out object? defaultValue);
             JsonNode parameterSchema = CreateJsonSchemaCore(
                 type: parameter.ParameterType,
                 parameter: parameter,
                 description: parameter.GetCustomAttribute<DescriptionAttribute>(inherit: true)?.Description,
-                hasDefaultValue: parameter.HasDefaultValue,
-                defaultValue: GetDefaultValueNormalized(parameter),
+                hasDefaultValue: hasDefaultValue,
+                defaultValue: defaultValue,
                 serializerOptions,
                 inferenceOptions);
 
             parameterSchemas.Add(parameter.Name, parameterSchema);
-            if (!parameter.IsOptional)
+            if (!parameter.IsOptional && !hasDefaultValue)
             {
                 (requiredProperties ??= []).Add((JsonNode)parameter.Name);
             }
@@ -760,6 +761,32 @@ private static JsonElement ParseJsonElement(ReadOnlySpan<byte> utf8Json)
         return JsonElement.ParseValue(ref reader);
     }
 
+    /// <summary>
+    /// Tries to get the effective default value for a parameter, checking both C# default value syntax and DefaultValueAttribute.
+    /// </summary>
+    /// <param name="parameterInfo">The parameter to check.</param>
+    /// <param name="defaultValue">The default value if one exists.</param>
+    /// <returns><see langword="true"/> if the parameter has a default value; otherwise, <see langword="false"/>.</returns>
+    internal static bool TryGetEffectiveDefaultValue(ParameterInfo parameterInfo, out object? defaultValue)
+    {
+        // First check for DefaultValueAttribute
+        if (parameterInfo.GetCustomAttribute<DefaultValueAttribute>(inherit: true) is { } attr)
+        {
+            defaultValue = attr.Value;
+            return true;
+        }
+
+        // Fall back to the parameter's declared default value
+        if (parameterInfo.HasDefaultValue)
+        {
+            defaultValue = GetDefaultValueNormalized(parameterInfo);
+            return true;
+        }
+
+        defaultValue = null;
+        return false;
+    }
+
     [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)

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

@@ -61,6 +61,51 @@ public async Task Parameters_DefaultValuesAreUsedButOverridable_Async()
         AssertExtensions.EqualFunctionCallResults("hello hello", await func.InvokeAsync(new() { ["a"] = "hello" }));
     }
 
+    [Fact]
+    public async Task Parameters_DefaultValueAttributeIsRespected_Async()
+    {
+        // Test with null default value
+        AIFunction funcNull = AIFunctionFactory.Create(([DefaultValue(null)] string? text) => text ?? "was null");
+
+        // Schema should not list 'text' as required and should have default value
+        string schema = funcNull.JsonSchema.ToString();
+        Assert.Contains("\"text\"", schema);
+        Assert.DoesNotContain("\"required\"", schema);
+        Assert.Contains("\"default\":null", schema);
+
+        // Should be invocable without providing the parameter
+        AssertExtensions.EqualFunctionCallResults("was null", await funcNull.InvokeAsync());
+
+        // Should be overridable
+        AssertExtensions.EqualFunctionCallResults("hello", await funcNull.InvokeAsync(new() { ["text"] = "hello" }));
+
+        // Test with non-null default value
+        AIFunction funcValue = AIFunctionFactory.Create(([DefaultValue("default")] string text) => text);
+        schema = funcValue.JsonSchema.ToString();
+        Assert.DoesNotContain("\"required\"", schema);
+        Assert.Contains("\"default\":\"default\"", schema);
+
+        AssertExtensions.EqualFunctionCallResults("default", await funcValue.InvokeAsync());
+        AssertExtensions.EqualFunctionCallResults("custom", await funcValue.InvokeAsync(new() { ["text"] = "custom" }));
+
+        // Test with int default value
+        AIFunction funcInt = AIFunctionFactory.Create(([DefaultValue(42)] int x) => x * 2);
+        schema = funcInt.JsonSchema.ToString();
+        Assert.DoesNotContain("\"required\"", schema);
+        Assert.Contains("\"default\":42", schema);
+
+        AssertExtensions.EqualFunctionCallResults(84, await funcInt.InvokeAsync());
+        AssertExtensions.EqualFunctionCallResults(10, await funcInt.InvokeAsync(new() { ["x"] = 5 }));
+
+        // Test that DefaultValue attribute takes precedence over C# default value
+        AIFunction funcBoth = AIFunctionFactory.Create(([DefaultValue(100)] int y = 50) => y);
+        schema = funcBoth.JsonSchema.ToString();
+        Assert.DoesNotContain("\"required\"", schema);
+        Assert.Contains("\"default\":100", schema); // DefaultValue should take precedence
+
+        AssertExtensions.EqualFunctionCallResults(100, await funcBoth.InvokeAsync()); // Should use DefaultValue, not C# default
+    }
+
     [Fact]
     public async Task Parameters_MissingRequiredParametersFail_Async()
     {