Thanks for reporting the issue. I tried to mock File.AppendText the following way that works nicely:
Mock.Arrange(() => File.AppendText(Arg.AnyString));
Therefore, I found out that DoNothing modifier is not available for members that return a value. But this an optional modifier added to increase readability and without it things should work just fine. But, yes that goes into our product backlog and should be fixed for the next build.
Also, please make sure that you have MockClassAttribute on top of your class which is required for mocking framework members.
Hope this information helps.
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