Or just use java and benefit from mature libraries and frameworks, tooling and package management that isn't a piece of shit. Your insurance is the openjdk then rather than a thinly veiled promise not to sue from Microsoft.
That "thinly veiled promise not to sue" is backed by the full force of a legally binding promise recognized by the combined weight of the US and Euro legal systems. Microsoft is, by centuries of established law and court decisions, precluded from suing someone from using C# through Mono or any other API or framework.
Moreover, using Java is now riskier than using C#, as Oracle has changed the licensing terms for the language, the API, and the development tools to allocate to itself greater legal control over the Java IPs than Sun did (or which Microsoft reserves as to C#).
EDIT: Just so we're clear, to all the non-lawyers and non-legally savvy HNers: The promise not to sue is itself what is binding, as a consequence of more than two centuries of US and Euro law (see, e.g., the concept of promissory estoppel). The point of the promise was to clarify the permissive license to use Microsoft's IP as it relates to C#.
No its not actually. The scope of the legal agreement is undefined and may or may not encompass bits of the framework which mono implements or may implement in the future. So don't count on it, which is what our rather expensive lawyer told us.
Some portions of the framework are not covered by the promise but Miguel mentioned something about Xamarin still having some kind of protection the project had when it was at Novell. Does anybody have any information that can prove/disprove this?
>That "thinly veiled promise not to sue" is backed by the full force of a legally binding promise recognized by the combined weight of the US and Euro legal systems.
Have you got a source for that? AIUI under UK law the concept of promissory estoppel only exists in the context of an agreement in which the other party has received consideration. Not having paid MS anything, I don't see how that can apply here.
In the US, my understanding is that consideration is necessary for contract formation, but by contrast promissory estoppel applies only when there is no consideration. A quick perusal of Wikipedia's coverage of the topic  makes it look like the UK's treatment is roughly similar.