Joe Mayo posted a great 'frequently asked question' along with a very clear answer, to the C# FAQ recently....

Why did I receive the error: "The type or namespace '<namespace name>' does not exist in the class or namespace '<parent namespace>' (are you missing an assembly reference?)"

If you haven't found the FAQ yet, check it out.... submit your questions (please try to stick to common C# issues, not general support questions, as we are trying to only answer questions that would make good additions to the FAQ list) and comment on existing ones! Oh, and if you are a C# MVP and you'd like to contribute to this blog.... just let me know.