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Hi, I contribute here a C# port of your Either class (from Java).

Notes: I modified a bit the implementation of Either:
* The IsLeft/IsRight properties are backed by a boolean instead of comparing left/right values with 'empty' (default(T). This allows, for example, having 0 a valid right value for a Either<Exception, int> instance.
* I added InvalidOperationException guards when accessing Left and Right properties.