“Interface not implemented” when Returning Derived Type

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Unfortunately, the return type must match. What you are looking for is called ‘return type covariance’ and C# doesn’t support that.

http://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=90909

Eric Lippert, senior developer on C# Compiler team, mentions on his blog that they don’t plan to support return type covariance.

“That kind of variance is called
“return type covariance”. As I
mentioned early on in this series, (a)
this series is not about that kind of
variance, and (b) we have no plans to
implement that kind of variance in C#.
“

http://blogs.msdn.com/ericlippert/archive/2008/05/07/covariance-and-contravariance-part-twelve-to-infinity-but-not-beyond.aspx

It’s worth reading Eric’s articles on covariance and contravariance.

http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx

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