Java is a popular language & platform, having a huge ecosystem. Are there any features that C# 4.0 onwards and its platforms should have, to make you completely switch to C# programming?
Make it run on as many platforms as Java?
Otherwise it is completely irrelevant to us.
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Edit: For a discussion about Mono making .NET "cross platform", see https://softwareengineering.stackexchange.com/questions/20275/mono-is-frequently-used-to-say-yes-net-is-cross-platform-how-valid-is-that
Edit 2015: Microsoft has decided to open source the backend part of .NET, which will enable community to port this to even more platforms than Java runs on. I consider this the ultimate move against the oracle approach to Java evolution.
Edit 2019: .NET Core is now available for Linux, and therefore as Docker images. Our experiences so far has been very nice. We have a mission critical application which runs nicely in Kubernetes, and we have no particular preference for writing microservices in either .NET or Java.