How to mock IProcess to in a c# Unit test

Question

I have a C# method I want to mock the return value of in a unit test. My test infrastructure uses Autofac and AutoMock for dependency inversion/injection.

public System.Diagnostics.Abstractions.IProcess GetClientIProcess(int ProcessId)
{
    var temp = Process.GetProcessById(ProcessId);
    System.Diagnostics.Abstractions.IProcess clientIProcess = new System.Diagnostics.Abstractions.Process(temp);
    return clientIProcess;
}

All I want to achieve is to mock the return value here so that it will return a valid IProcess that is used in the real method that I am testing in my unit test

Answer 1

If you want to use Moq to create a Test Double version of IProcess, you'd create an object in your test code in this way:

var processTD = new Mock<IProcess>();

and then somehow pass processTD.Object to your System Under Test (SUT).

The processTD.Object object will be a particular implementation of IProcess with test-specific behaviour, specified by the test code. Only that object will have that behaviour. This object's concrete type is an auto-emitted class that implements the IProcess interface.

This (simplified) line of code in the current SUT:

IProcess clientIProcess = new Process(temp);

creates and returns a Process object. The Process class also implements the IProcess interface, but it is not a Moq object, and neither can you change it to become a Moq object as long as you new it up as Process.

If you want to test using Moq, you'll have to figure out how to hook into the SUT in a way that enables you to replace one IProcess object with another.

A common way to do that is via Dependency Injection, but you could also use a design pattern like Factory Method.