Poser

Poser allows you to replace any .NET method (including static and non-virtual) with a delegate. It is similar to Microsoft Fakes but unlike it Poser 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.

Poser 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.

Installation

Available on NuGet

Visual Studio:

PM> Install-Package Poser

.NET Core CLI:

dotnet add package Poser

Usage

Poser 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 PoseContext class.

Shim static method

using Pose;

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

Shim static property getter

using Pose;

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

Shim static property setter

using Pose;

Shim setterShim = Shim.Replace(() => Console.Title, true).With((string title) => { Console.Title = "My Title"; });

Shim instance property getter

using Pose;

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);

Shim instance property setter

using Pose;

Shim classPropShim = Shim.Replace(() => Is.A<MyClass>().MyProperty, true).With((MyClass @this, int prop) => { @this.MyProperty = prop * 10; });

Shim constructor

using Pose;

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

Shim instance method of a Reference Type

using Pose;

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

Shim method of specific instance of a Reference Type

using Pose;

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

Shim instance method of a Value Type

using Pose;

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

Shim operators

var operatorShim = Shim.Replace(() => Is.A<TimeSpan>() + Is.A<TimeSpan>()).With(
    delegate(TimeSpan l, TimeSpan r) { return TimeSpan.Zero; });

Isolating your code

// This block executes immediately
PoseContext.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();
    
    // Outputs '00:00:00'
    Console.WriteLine(TimeSpan.FromDays(1) + TimeSpan.FromSeconds(2));

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

Shimming operators

Operator shimming requires that the class/struct overloads the operator in question.

Poser supports shimming operators of the following kind:

• Arithmetic
  • +x
  • -x
  • !x
  • ~x
  • x + y
  • x - y
  • x / y
  • x % y
  • x & y
  • x | y
  • x ^ y
  • x << y
  • x >> y
• Equality
  • x == y
  • x != y
• Comparison
  • x < y
  • x > y
  • x <= y
  • x >= y

In addition to this, both implicit and explicit conversion operators are supported.

Unsupported operators

Shimming of the following operators is not supported:

• true and false because I cannot find a good way to express the operation in an expression tree.
• x >>> y because expression trees cannot contain this operator. This is a limitation on the part of the compiler.
• ++ and -- because these cannot be expressed in an expression tree.

Caveats & Limitations

• 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.

Roadmap

• Performance Improvements - Poser 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.

Issues & Contributions

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.

License

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