Not sure that I can see much value in it if the response to a failure is just a compiler error message. The response should explain why the submitted solution is wrong and give a suggested solution with an explanation of why it is to be preferred.

For instance this compiles in Visual Studio 2015 but fails on Coding Monkey:

    public class StringManipulation
    {
        public string GetFirstLetter(string input)
        {
            return input.SubString(0, 1);
        }
    }

with this error:

CS1061 5 'string' does not contain a definition for 'SubString' and no extension method 'SubString' accepting a first argument of type 'string' could be found (are you missing a using directive or an assembly reference?)

What's going on? MSDN says that string is an alias for System.String which definitely has SubString().

I don't use C# very much but I do use .Net very heavily so I am very interested to know why that code doesn't work on Coding Monkey. Perhaps it will improve my code.

Edit: I do realize that explaining why something is wrong is a seriously difficult task, but eventually the same wrong answers will appear repeatedly so a canned response should be possible in the end, at least for some code.