public static class AutoMapperTestHelper
{
    public static void ShouldFormatValueAs<TInput>(
        this IValueFormatter formatter, TInput input, string expectedOutput)
    {
        ResolutionContext rc = new ResolutionContext(null, input, 
            typeof(TInput), typeof(string));

        string actualOutput = formatter.FormatValue(rc);

        if (!actualOutput.Equals(expectedOutput))
        {
            throw new AutoMapperOutputAssertionException(String.Format(
                "Expected IValueFormatter output: {0}. Actual output: {1}", 
                expectedOutput, actualOutput));
        }
    }
}

public class AutoMapperOutputAssertionException : Exception
{
    public AutoMapperOutputAssertionException(string message)
		: base(message) { }
}