Question

I have written a unit test with MD 2.8.5 in a project that includes System.Core and with build target Mono/.NET 3.5. I really like the Assert.Throws of the newer NUnit, so decided to write an extension method for it. I created a new file with this as its content in the same namespace as the test. Can anyone see my error?

public delegate void TestDelegate();

public static class AssertThrows
{
    public static T Throws<T>(this Assert assert, TestDelegate td)
        where T : Exception
    {
        try
        {
            td();
        }
        catch(T e)
        {
            return e;
        }
        catch
        {
            throw new AssertionException("Wrong exception type.");
        }
        throw new AssertionException("Did not throw an error.");
    }
}

MonoDevelop “sees” the extension method through its code completion. However, the compiler reports:

Performing main compilation...
/Users/shamwow/dev/EngineTests.cs(19,37): error CS0117:
  `NUnit.Framework.Assert' does not  contain a definition for `Throws'
/Applications/MonoDevelop.app/Contents/MacOS/lib/monodevelop/AddIns/NUnit/nunit.framework.dll (Location of the symbol related to previous error)


Build complete -- 1 error, 0 warnings

(I know MD and Mono are not the same.)

Answer 1

I assume you’re trying to use it just as:

Assert.Throws<FooException>(() => ...);

Extension methods don’t work like that – they appear to be instance methods on the extended type. As you won’t have an instance of Assert, you can’t call your extension method like that.