Supporting C#9 init-only properties for Commands

[zlavoie]

Is your feature request related to a problem? Please describe.
C#9 came out with records and init-only properties. So now when trying to implement records, I am limited to using 'set' in IRequest.

Describe the solution you'd like
If the Id of IRequest can be init instead of set, then it can be used in a record and the comand can be fired to the handler.

Describe alternatives you've considered
I could use set, and then that is the only property that is NOT immutable, which starts to violate the purpose of a record in C#9.

[iancooper]

Yeah, we need to look at how we support C# 9 and even nullable reference types properly. One issue for us is backwards compatibility. For now, in a related example, it may make more sense for us to target LTS releases of .NET, so 6 vs 5. When looking at new language feature support i.e. treating a message as a record type, we need to think about breaking folks who are not using C# 9 yet.

So it needs some thought to understand their experience

[iancooper]

@zlavoie Do you know anything about the backward compatibility of C# 9 init properties, I have not had a chance to look