C# language support

anonymous functions
operators (including user-defined operator overloading)
await on Task, Task<TResult>
conditional access (?. operator)
assignments (including composite assignment operators)
literals
ref and out parameters
optional parameters, default value
delegates (having Combine, Remove, Target)
implicit and explicit casts (including user-defined implicit/explicit operators), missing: numeric explicit casts (see planned) properly truncating value
collection initializers (including dictionary initializers)
anonymous objects
query expressions - from ... select ...
string interpolation (missing: padding)
control flow: labels/goto, break, continue, return, throw, yield return, while, do/while, for, foreach, using, if, switch, try/catch/finally
generator methods (with limitations: see planned)
async methods returning Task, Task<TResult> (with limitations: see planned)

These features and fixes are planned for stable release.

struct definitions
numeric explicit casts truncating value (currently does nothing, it is recommended to use System.Math methods instead)
checked/unchecked
padding in string interpolation
resolving current limitations of generator and async methods: block-scoped variables and try/finally does not currently work inside a method transformed to a state-machine
nullable operators should be absorbing (null + 1 equals null in .NET)

These keywords are used for interoperability with unmanage code or in multi-threaded environment which is not