I have on several occasions wished that I could setup expectations in Moq for subsequent calls to a method.  For example, I might want Moq to return one value the first time a method is called, but a different value the second and third times.  Phil Haack has a nice way to achieve this, but his implementation only allows you to return a result.  What if you need something more advanced?  What if you want to return a value on the first call, throw an exception on the second, and return a value again on the third?  I ran into exactly this situation while testing out some error-handling logic in a class.  Here’s how I achieved it.

public static class MoqExtensions
{
    public static IReturnsResult<TMockType> Returns<TMockType, TReturnType>(this IReturns<TMockType, TReturnType> expression, params Func<TReturnType>[] results) 
        where TMockType : class
    {
        int nextResult = 0;

        return expression.Returns(() => results[nextResult++]());
    }
}

This adds a new Returns method you can use with Moq in place of one of the built-in Return methods.  It allows you to specify an arbitrary number of Func’s to be executed.  Instead of using Moq to return the Func’s directly, it creates a new Func (via a lambda expression) that will iterate over the result Func’s specified.  It does this via a closure around the results and the index of the next result.  Note that it accepts a ‘params’ array, so calling it still looks (fairly) clean:

[Test]
public void ContinuesWorkingIfErrorEventIsHandled()
{
    mResultSet.Setup(r => r.GetNextResult()).Returns((Func<IResult>)(() => { throw new NotImplementedException(); }), () => null);

    //Run...
    
    mResultSet.Verify(r => r.GetNextResult(), Times.Exactly(2));
}

That’s all there is to it!

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