UnsafeCLR

UnsafeCLR is a .NET library that allows for runtime method replacement (also known as method hooking or patching) on the Common Language Runtime (CLR). This can be used for advanced scenarios such as mocking, hot-patching, or instrumentation—without recompilation or modifying original source code.

⚠️ This library uses unsafe and low-level operations. Use it with caution and only when absolutely necessary.

Features

• Replace instance methods at runtime
• Replace static methods at runtime
• Minimal and straightforward API
• Compatible with modern .NET runtimes (tested on .NET 6+)

Limitations

• Inlined methods will not be replaced, or the replacement could be permanant since the code of the called method will be baked into the caller. (A workaround is to disable JIT inlining: COMPlus_JitNoInline=1)
• .NET 6 on ARM64 is not supported.

Getting Started

Installation

dotnet add package UnsafeCLR

API

UnsafeCLR.ReplaceInstanceMethod

public static MethodReplacement ReplaceInstanceMethod(
    Type originalInstanceType,
    MethodInfo originalMethod,
    MethodInfo replacingMethod
)

• Replaces an instance method at runtime.

UnsafeCLR.ReplaceStaticMethod

public static MethodReplacement ReplaceStaticMethod(
    MethodInfo originalMethod,
    MethodInfo replacingMethod
)

• Replaces a static method at runtime.

Warnings

• This library uses low-level patching and may break with future CLR versions.
• Not suitable for use in high-security or production-critical environments.
• Ensure method signatures match exactly (return types and parameters).

Example Use Cases

• Mocking static methods
• Hot-fixing logic in running apps (debugging/instrumentation)

Example: Replace an Instance Method

using UnsafeCLR;
using System.Reflection;

class Program {
    static MethodInfo GetMethodInfo<T, TRes>(Func<T, TRes> fun) => fun.Method;
    static MethodInfo GetMethodInfo<T1, T2, TRes>(Func<T1, T2, TRes> fun) => fun.Method;
    
    public class MyClass {
        public int MyFunc(int param1) {
            // original code
            return param1 + param1;
        }
    }

    public static int MyReplacement(MyClass instance, int param1) {
        // replacing code;
        return param1 * param1;
    }

    public static void Main() {
        var instance = new MyClass();
        using (var replacement = CLRHelper.ReplaceInstanceMethod(typeof(MyClass), GetMethodInfo<int, int>(instance.MyFunc), GetMethodInfo<MyClass, int, int>(MyReplacement))) {
            Console.WriteLine("Inside 'using'");
            Console.WriteLine($"instance.MyFunc = {instance.MyFunc(10)}");
            Console.WriteLine($"MyReplacement = {MyReplacement(instance, 10)}");
            Console.WriteLine($"OriginalMethod.Invoke = { (int) replacement.OriginalMethod.Invoke(null, new object[] { instance, 10 }) }");
        }
        Console.WriteLine("Outside 'using'");
        Console.WriteLine($"instance.MyFunc = {instance.MyFunc(10)}");
        Console.WriteLine($"MyReplacement = {MyReplacement(instance, 10)}");
    }
}

License

See LICENSE.md