Returning value that was passed into a method

Even more useful, if you have multiple parameters you can access any/all of them with:

_mock.Setup(x => x.DoSomething(It.IsAny<string>(),It.IsAny<string>(),It.IsAny<string>())
     .Returns((string a, string b, string c) => string.Concat(a,b,c));

You always need to reference all the arguments, to match the method's signature, even if you're only going to use one of them.

You can use a lambda with an input parameter, like so:

.Returns((string myval) => { return myval; });

Or slightly more readable:

.Returns<string>(x => x);

The generic Returns<T> method can handle this situation nicely.

_mock.Setup(x => x.DoSomething(It.IsAny<string>())).Returns<string>(x => x);

Or if the method requires multiple inputs, specify them like so:

_mock.Setup(x => x.DoSomething(It.IsAny<string>(), It.IsAny<int>())).Returns((string x, int y) => x);