NShim allows you to replace any .NET method (including static and non-virtual) with a delegate. It is similar to Microsoft Fakes but unlike it NShim is implemented entirely in managed code (Reflection Emit API). Everything occurs at runtime and in-memory, no unmanaged Profiling APIs and no file system pollution with re-written assemblies.

NShim is cross platform and runs anywhere .NET is supported. It targets .NET Standard 2.0 so it can be used across .NET platforms including .NET Framework, .NET Core, Mono and Xamarin. See version compatibility table here.

Available on NuGet

Visual Studio:

PM> Install-Package NShim

.NET Core CLI:

dotnet add package NShim

NShim gives you the ability to create shims by way of the Shim class. Shims are basically objects that let you specify the method you want to replace as well as the replacement delegate. Delegate signatures (arguments and return type) must match that of the methods they replace. The Is class is used to create instances of a type and all code you want to apply your shims to is isolated using the NShimContext class.

using NShim;

Shim consoleShim = Shim.Replace(() => Console.WriteLine(Is.A<string>())).With(
    delegate (string s) { Console.WriteLine("Hijacked: {0}", s); });

using NShim;

Shim dateTimeShim = Shim.Replace(() => DateTime.Now).With(() => new DateTime(2004, 4, 4));

using NShim;

class MyClass
{
    public int MyProperty { get; set; }
    public void DoSomething() => Console.WriteLine("doing someting");
}

Shim classPropShim = Shim.Replace(() => Is.A<MyClass>().MyProperty).With((MyClass @this) => 100);

using NShim;

Shim ctorShim = Shim.Replace(() => new MyClass()).With(() => new MyClass() { MyProperty = 10 });

using NShim;

Shim classShim = Shim.Replace(() => Is.A<MyClass>().DoSomething()).With(
    delegate (MyClass @this) { Console.WriteLine("doing someting else"); });

Note: The first argument to an instance method replacement delegate is always the instance of the class

using NShim;

MyClass myClass = new MyClass();
Shim myClassShim = Shim.Replace(() => myClass.DoSomething()).With(
    delegate (MyClass @this) { Console.WriteLine("doing someting else with myClass"); });

using NShim;

Shim structShim = Shim.Replace(() => Is.A<MyStruct>().DoSomething()).With(
    delegate (ref MyStruct @this) { Console.WriteLine("doing someting else"); });

Note: You cannot shim methods on specific instances of Value Types

// This block executes immediately
NShimContext.Isolate(() =>
{
    // All code that executes within this block
    // is isolated and shimmed methods are replaced

    // Outputs "Hijacked: Hello World!"
    Console.WriteLine("Hello World!");

    // Outputs "4/4/04 12:00:00 AM"
    Console.WriteLine(DateTime.Now);

    // Outputs "doing someting else"
    new MyClass().DoSomething();

    // Outputs "doing someting else with myClass"
    myClass.DoSomething();

}, consoleShim, dateTimeShim, classPropShim, classShim, myClassShim, structShim);

• Breakpoints - At this time any breakpoints set anywhere in the isolated code and its execution path will not be hit. However, breakpoints set within a shim replacement delegate are hit.
• Exceptions - At this time all unhandled exceptions thrown in isolated code and its execution path are always wrapped in System.Reflection.TargetInvocationException.

• Performance Improvements - NShim can be used outside the context of unit tests. Better performance would make it suitable for use in production code, possibly to override legacy functionality.
• Exceptions Stack Trace - Currently when exceptions are thrown in your own code under isolation, the supplied exception stack trace is quite confusing. Providing an undiluted exception stack trace is needed.

If you find a bug or have a feature request, please report them at this repository's issues section. Contributions are highly welcome, however, except for very small changes kindly file an issue and let's have a discussion before you open a pull request.

This project is licensed under the MIT license. See the LICENSE file for more info.

NShim is a rewrite of and heavily inspired by Pose.