Moq: How to get to a parameter passed to a method of a mocked service

You can use the Mock.Callback-method:

var mock = new Mock<Handler>();
SomeResponse result = null;
mock.Setup(h => h.AsyncHandle(It.IsAny<SomeResponse>()))
    .Callback<SomeResponse>(r => result = r);

// do your test
new Foo(mock.Object).Bar(22);
Assert.NotNull(result);

If you only want to check something simple on the passed in argument, you also can do it directly:

mock.Setup(h => h.AsyncHandle(It.Is<SomeResponse>(response => response != null)));

The Callback method will certainly work, but if you are doing this on a method with a lot of parameters it can be a bit verbose. Here is something that I have used to remove some of the boilerplate.

var mock = new Mock<Handler>();

// do your test   
new Foo(mock.Object).Bar(22);

var arg = new ArgumentCaptor<SomeResponse>();
mock.Verify(h => h.AsyncHandle(arg.Capture()));
Assert.NotNull(arg.Value);

Here is the source for ArgumentCaptor:

public class ArgumentCaptor<T>
{
    public T Capture()
    {
        return It.Is<T>(t => SaveValue(t));
    }

    private bool SaveValue(T t)
    {
        Value = t;
        return true;
    }

    public T Value { get; private set; }
}

An alternative is to use Capture.In, which is an out-of-the-box feature in Moq that lets you capture arguments into a collection:

//Arrange
var args = new List<SomeResponse>();
mock.Setup(h => h.AsyncHandle(Capture.In(args)));

//Act
new Foo(mock.Object).Bar(22);

//Assert
//... assert args.Single() or args.First()

Gamlor's answer worked for me, but I thought I would expand on John Carpenter's comment because I was looking for a solution involving more than one parameter. I figured other folks who stumble onto this page may be in a similar situation. I found this info in the Moq documentation.

I'll use Gamlor's example, but let's pretend the AsyncHandle method takes two arguments: a string and a SomeResponse object.

var mock = new Mock<Handler>();
string stringResult = string.Empty;
SomeResponse someResponse = null;
mock.Setup(h => h.AsyncHandle(It.IsAny<string>(), It.IsAny<SomeResponse>()))
    .Callback<string, SomeResponse>((s, r) => 
    {
        stringResult = s;
        someResponse = r;
    });

// do your test
new Foo(mock.Object).Bar(22);
Assert.AreEqual("expected string", stringResult);
Assert.IsNotNull(someResponse);

Basically you just need to add another It.IsAny<>() with the appropriate type, add another type to the Callback method, and change the lambda expression as appropriate.