不幸的是，返回类型必须匹配。您正在寻找的被称为返回类型协方差什么和C＃不支持。

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

埃里克利珀，在C＃编译器团队的高级开发人员，提到在他的博客，他们不打算支持的返回类型协方差。

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

这种差异被称为
  返回类型协方差。正如我
  在这个系列中提到的早期，（一）
  这个系列是不是那种
  方差，和（b）我们没有计划
  C＃实现那种方差。
  

"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

这是值得一读的协变和逆变Eric的文章。

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

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