.Net: Expose underlying method from kernel function (#11378)

### Motivation, Context and Description
Currently, there is no way to access the underlying method that an
instance of `KernelFunctionFromMethod` was created from or wraps. Being
able to access this underlying method is useful when additional metadata
is needed. For example, if the method has a custom attribute, you may
need to retrieve that attribute to determine the execution path.
   
This PR adopts the same approach as M.E.AI by exposing the native method
through the `public MethodInfo? UnderlyingMethod {...}` property. This
property is null by default and is only initialized for native functions
represented by the `KernelFunctionFromMethod` class.

Closes: #11182

---------

Co-authored-by: Roger Barreto <19890735+RogerBarreto@users.noreply.github.com>

Author: SergeyMenshykh

dotnet/samples/Concepts/Functions/MethodFunctions_Advanced.cs

@@ -2,27 +2,28 @@
 
 using System.ComponentModel;
 using System.Globalization;
+using System.Reflection;
 using System.Text.Json;
 using Microsoft.SemanticKernel;
 
 namespace Functions;
 
-// This example shows different ways how to define and execute method functions using custom and primitive types.
+/// <summary>
+/// These samples show advanced usage of method functions.
+/// </summary>
 public class MethodFunctions_Advanced(ITestOutputHelper output) : BaseTest(output)
 {
-    #region Method Functions Chaining
-
     /// <summary>
     /// This example executes Function1, which in turn executes Function2.
     /// </summary>
     [Fact]
-    public async Task MethodFunctionsChainingAsync()
+    public async Task MethodFunctionsChaining()
     {
         Console.WriteLine("Running Method Function Chaining example...");
 
         var kernel = new Kernel();
 
-        var functions = kernel.ImportPluginFromType<FunctionsChainingPlugin>();
+        var functions = kernel.ImportPluginFromType<Plugin>();
 
         var customType = await kernel.InvokeAsync<MyCustomType>(functions["Function1"]);
 
@@ -31,11 +32,28 @@ public async Task MethodFunctionsChainingAsync()
     }
 
     /// <summary>
-    /// Plugin example with two method functions, where one function is called from another.
+    /// This example shows how to access the custom <see cref="InvocationSettingsAttribute"/> attribute the underlying method wrapped by Kernel Function is annotated with.
     /// </summary>
-    private sealed class FunctionsChainingPlugin
+    [Fact]
+    public async Task AccessUnderlyingMethodAttributes()
+    {
+        // Import the plugin containing the method with the InvocationSettingsAttribute custom attribute
+        var kernel = new Kernel();
+
+        var functions = kernel.ImportPluginFromType<Plugin>();
+
+        // Get the kernel function wrapping the method with the InvocationSettingsAttribute
+        var kernelFunction = functions[nameof(Plugin.FunctionWithInvocationSettingsAttribute)];
+
+        // Access the custom attribute the underlying method is annotated with
+        var invocationSettingsAttribute = kernelFunction.UnderlyingMethod!.GetCustomAttribute<InvocationSettingsAttribute>();
+
+        Console.WriteLine($"Priority: {invocationSettingsAttribute?.Priority}");
+    }
+
+    private sealed class Plugin
     {
-        private const string PluginName = nameof(FunctionsChainingPlugin);
+        private const string PluginName = nameof(Plugin);
 
         [KernelFunction]
         public async Task<MyCustomType> Function1Async(Kernel kernel)
@@ -59,11 +77,12 @@ public static MyCustomType Function2()
                 Text = "From Function2"
             };
         }
-    }
-
-    #endregion
 
-    #region Custom Type
+        [KernelFunction, InvocationSettingsAttribute(priority: Priority.High)]
+        public static void FunctionWithInvocationSettingsAttribute()
+        {
+        }
+    }
 
     /// <summary>
     /// In order to use custom types, <see cref="TypeConverter"/> should be specified,
@@ -110,5 +129,20 @@ private sealed class MyCustomTypeConverter : TypeConverter
         }
     }
 
-    #endregion
+    [AttributeUsage(AttributeTargets.Method)]
+    private sealed class InvocationSettingsAttribute : Attribute
+    {
+        public InvocationSettingsAttribute(Priority priority = Priority.Normal)
+        {
+            this.Priority = priority;
+        }
+
+        public Priority Priority { get; }
+    }
+
+    private enum Priority
+    {
+        Normal,
+        High,
+    }
 }

dotnet/src/SemanticKernel.Abstractions/Functions/KernelFunction.cs

@@ -8,6 +8,7 @@
 using System.Diagnostics.CodeAnalysis;
 using System.Diagnostics.Metrics;
 using System.Linq;
+using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Text.Json;
 using System.Threading;
@@ -98,6 +99,15 @@ public abstract class KernelFunction
     /// </remarks>
     public IReadOnlyDictionary<string, PromptExecutionSettings>? ExecutionSettings { get; }
 
+    /// <summary>
+    /// Gets the underlying <see cref="MethodInfo"/> that this function might be wrapping.
+    /// </summary>
+    /// <remarks>
+    /// Provides additional metadata on the function and its signature. Implementations not wrapping .NET methods may return null.
+    /// </remarks>
+    [Experimental("SKEXP0001")]
+    public MethodInfo? UnderlyingMethod { get; internal init; }
+
     /// <summary>
     /// Initializes a new instance of the <see cref="KernelFunction"/> class.
     /// </summary>

dotnet/src/SemanticKernel.Core/Functions/KernelFunctionFromMethod.cs

@@ -141,6 +141,7 @@ public static KernelFunction Create(
 
         MethodDetails methodDetails = GetMethodDetails(options?.FunctionName, method, target);
         var result = new KernelFunctionFromMethod(
+            method,
             methodDetails.Function,
             methodDetails.Name,
             options?.Description ?? methodDetails.Description,
@@ -181,6 +182,7 @@ public static KernelFunction Create(
 
         MethodDetails methodDetails = GetMethodDetails(options?.FunctionName, method, jsonSerializerOptions, target);
         var result = new KernelFunctionFromMethod(
+            method,
             methodDetails.Function,
             methodDetails.Name,
             options?.Description ?? methodDetails.Description,
@@ -275,6 +277,7 @@ public static KernelFunctionMetadata CreateMetadata(
 
         MethodDetails methodDetails = GetMethodDetails(options?.FunctionName, method, null);
         var result = new KernelFunctionFromMethod(
+            method,
             methodDetails.Function,
             methodDetails.Name,
             options?.Description ?? methodDetails.Description,
@@ -307,6 +310,7 @@ public static KernelFunctionMetadata CreateMetadata(
 
         MethodDetails methodDetails = GetMethodDetails(options?.FunctionName, method, jsonSerializerOptions, target: null);
         var result = new KernelFunctionFromMethod(
+            method,
             methodDetails.Function,
             methodDetails.Name,
             options?.Description ?? methodDetails.Description,
@@ -382,6 +386,7 @@ public override KernelFunction Clone(string pluginName)
         if (base.JsonSerializerOptions is not null)
         {
             return new KernelFunctionFromMethod(
+            this.UnderlyingMethod!,
             this._function,
             this.Name,
             pluginName,
@@ -397,6 +402,7 @@ public override KernelFunction Clone(string pluginName)
         KernelFunctionFromMethod Clone()
         {
             return new KernelFunctionFromMethod(
+            this.UnderlyingMethod!,
             this._function,
             this.Name,
             pluginName,
@@ -424,31 +430,34 @@ private record struct MethodDetails(string Name, string Description, Implementat
     [RequiresUnreferencedCode("Uses reflection to handle various aspects of the function creation and invocation, making it incompatible with AOT scenarios.")]
     [RequiresDynamicCode("Uses reflection to handle various aspects of the function creation and invocation, making it incompatible with AOT scenarios.")]
     private KernelFunctionFromMethod(
+        MethodInfo method,
         ImplementationFunc implementationFunc,
         string functionName,
         string description,
         IReadOnlyList<KernelParameterMetadata> parameters,
         KernelReturnParameterMetadata returnParameter,
         ReadOnlyDictionary<string, object?>? additionalMetadata = null) :
-        this(implementationFunc, functionName, null, description, parameters, returnParameter, additionalMetadata)
+        this(method, implementationFunc, functionName, null, description, parameters, returnParameter, additionalMetadata)
     {
     }
 
     private KernelFunctionFromMethod(
+        MethodInfo method,
         ImplementationFunc implementationFunc,
         string functionName,
         string description,
         IReadOnlyList<KernelParameterMetadata> parameters,
         KernelReturnParameterMetadata returnParameter,
         JsonSerializerOptions jsonSerializerOptions,
         ReadOnlyDictionary<string, object?>? additionalMetadata = null) :
-        this(implementationFunc, functionName, null, description, parameters, returnParameter, jsonSerializerOptions, additionalMetadata)
+        this(method, implementationFunc, functionName, null, description, parameters, returnParameter, jsonSerializerOptions, additionalMetadata)
     {
     }
 
     [RequiresUnreferencedCode("Uses reflection to handle various aspects of the function creation and invocation, making it incompatible with AOT scenarios.")]
     [RequiresDynamicCode("Uses reflection to handle various aspects of the function creation and invocation, making it incompatible with AOT scenarios.")]
     private KernelFunctionFromMethod(
+        MethodInfo method,
         ImplementationFunc implementationFunc,
         string functionName,
         string? pluginName,
@@ -461,9 +470,11 @@ private KernelFunctionFromMethod(
         Verify.ValidFunctionName(functionName);
 
         this._function = implementationFunc;
+        this.UnderlyingMethod = method;
     }
 
     private KernelFunctionFromMethod(
+        MethodInfo method,
         ImplementationFunc implementationFunc,
         string functionName,
         string? pluginName,
@@ -477,6 +488,7 @@ private KernelFunctionFromMethod(
         Verify.ValidFunctionName(functionName);
 
         this._function = implementationFunc;
+        this.UnderlyingMethod = method;
     }
 
     [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access otherwise can break functionality when trimming application code", Justification = "This method is AOT save.")]

dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionFromMethodTests2.cs

@@ -288,6 +288,24 @@ public void ItMakesProvidedExtensionPropertiesAvailableViaMetadataWhenConstructe
         Assert.Equal("value1", func.Metadata.AdditionalProperties["key1"]);
     }
 
+    [Fact]
+    public void ItShouldExposeUnderlyingMethod()
+    {
+        // Arrange
+        var target = new LocalExamplePlugin();
+
+        var methodInfo = target.GetType().GetMethod(nameof(LocalExamplePlugin.FunctionWithCustomAttribute))!;
+
+        var kernelFunction = KernelFunctionFactory.CreateFromMethod(methodInfo, target);
+
+        // Assert
+        Assert.NotNull(kernelFunction.UnderlyingMethod);
+
+        Assert.Equal(methodInfo, kernelFunction.UnderlyingMethod);
+
+        Assert.NotNull(kernelFunction.UnderlyingMethod.GetCustomAttribute<CustomAttribute>());
+    }
+
     private interface IExampleService;
 
     private sealed class ExampleService : IExampleService;
@@ -466,6 +484,11 @@ public string WithPrimitives(
         {
             return string.Empty;
         }
+
+        [KernelFunction, CustomAttribute]
+        public void FunctionWithCustomAttribute()
+        {
+        }
     }
 
     private sealed class GenericPlugin<T>
@@ -479,4 +502,9 @@ private sealed class GenericPlugin<T>
         [KernelFunction]
         public Task<T> GetValue3Async(T input) => Task.FromResult(input);
     }
+
+    [AttributeUsage(AttributeTargets.Method)]
+    private sealed class CustomAttribute : Attribute
+    {
+    }
 }