How Can Reflection Be Used to Identify Extension Methods in C#?

Using Reflection to Identify Extension Methods

In C#, extension methods provide a convenient way to add new functionality to existing types without modifying the original class definition. Determining whether an extension method has been added to a class can be useful in various scenarios, such as unit testing or ensuring proper implementation.

To identify extension methods using reflection, you must first look in the assemblies where the extension method may be defined. Identify classes decorated with the ExtensionAttribute attribute and examine the methods within those classes. Any method marked with the ExtensionAttribute and whose first parameter type matches the type you're investigating is an extension method.

using System;
using System.Runtime.CompilerServices;
using System.Reflection;
using System.Linq;
using System.Collections.Generic;

public static class FirstExtensions
{
    public static void Foo(this string x) { }
    public static void Bar(string x) { } // Not an ext. method
    public static void Baz(this int x) { } // Not on string
}

public static class SecondExtensions
{
    public static void Quux(this string x) { }
}

public class Test
{
    static void Main()
    {
        Assembly thisAssembly = typeof(Test).Assembly;
        foreach (MethodInfo method in GetExtensionMethods(thisAssembly, typeof(string)))
        {
            Console.WriteLine(method);
        }
    }

    static IEnumerable<MethodInfo> GetExtensionMethods(Assembly assembly, Type extendedType)
    {
        var isGenericTypeDefinition = extendedType.IsGenericType &amp;&amp; extendedType.IsTypeDefinition;
        var query = from type in assembly.GetTypes()
            where type.IsSealed &amp;&amp; !type.IsGenericType &amp;&amp; !type.IsNested
            from method in type.GetMethods(BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic)
            where method.IsDefined(typeof(ExtensionAttribute), false)
            where isGenericTypeDefinition
                ? method.GetParameters()[0].ParameterType.IsGenericType &amp;&amp; method.GetParameters()[0].ParameterType.GetGenericTypeDefinition() == extendedType
                : method.GetParameters()[0].ParameterType == extendedType
            select method;
        return query;
    }
}

In this example, GetExtensionMethods identifies all extension methods for the string type in the thisAssembly assembly. It gathers types decorated with ExtensionAttribute and examines their methods for the appropriate parameter type and ExtensionAttribute.

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