Moq setting method return value

I think that you need to change the Returns to just return true instead of the lambda. Like this:

rabbitConection.Setup(e => e.AddToQueue(null, string.Empty)).Returns(true)

EDIT:

If this still doesn’t work then it is probably due to the parameters not matching. You are passing in "someExchange" but the mock is set up for string.Empty. If you aren’t sure what values will be used you could use the It.IsAny method to get around this.

rabbitConection.Setup(e => e.AddToQueue(It.IsAny<byte[]>(), It.IsAny<string>())).Returns(true)