Whilst no, the Microsoft CLR is not open source. It doesn't need to be. Because the CLI is a ECMA standard and known-good third party implementations (Mono) already exist.
The only thing to be gained from open sourcing the Microsoft CLR would be to benefit Mono. As Mono is the defacto official cross-platform port of the CLR. Since the Microsoft CLR almost certainly contains IP and trade secrets that they wouldn't want their commercial rivals to see, it would be more beneficial if Microsoft merely gave a fairly open code sharing and IP license to the Mono / Xamarin team so that they could benefit in the exact same way as though it were OSS MIT.
Whilst no, the Microsoft CLR is not open source. It doesn't need to be. Because the CLI is a ECMA standard and known-good third party implementations (Mono) already exist.
The only thing to be gained from open sourcing the Microsoft CLR would be to benefit Mono. As Mono is the defacto official cross-platform port of the CLR. Since the Microsoft CLR almost certainly contains IP and trade secrets that they wouldn't want their commercial rivals to see, it would be more beneficial if Microsoft merely gave a fairly open code sharing and IP license to the Mono / Xamarin team so that they could benefit in the exact same way as though it were OSS MIT.