Champion "Allow no-arg constructor and field initializers in struct declarations"

[gafter]

• Proposal added: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/parameterless-struct-constructors.md
• Discussed in LDM
• Decision in LDM
• Finalized: Done
• Spec'ed

See also dotnet/roslyn#13921 dotnet/roslyn#1029

LDM history:

• https://github.com/dotnet/csharplang/blob/master/meetings/2021/LDM-2021-01-27.md#field-initializers

[iam3yal]

@jnm2 I don't think that it's about the benefit but more of a consistency so yes symmetry but dunno what it means for the run-time, I'm not sure what would happen when you define a parameterless constructor and set the fields, I voted because I hope that we could have default values but dunno.

[DavidArno]

Downvoting this as per my comment for Roslyn #13921. For a struct, S, if default(S) and new S() leads to different values, then this feature has no real benefit and will cause serious bugs. This should be implemented properly in the CLR before it's considered for C#.

[orthoxerox]

I agree. If we're going to get non-nullable reference types and structs with parameterless constructors, then we should be able to override default for them,

[iam3yal]

I really hope that they will design and implement this properly.

[HaloFour]

To play Devil's advocate, why is it expected that new struct() and default(struct) produce the same value? That's never been the case with reference types.

Does C# support consuming structs with default constructors today? You could always define one using IL or some other language. How about the IL that C# produces for a generic method with a new constraint? Would that work today with such a type?

The concept of a "default" for struct types is interesting but even in the case of a blittable default you're talking about adding some serious overhead.

[iam3yal]

@HaloFour Well, it might be possible to mitigate the overhead by an attribute? like opt-in/out?

[DavidArno]

@HaloFour,

Your devil's advocate is a good question. To my mind, the answer is "and that is exactly what's wrong with reference types". It is the driver behind non-nullable reference types. SO just at the time when we are looking to address the "null problem", we'd be introducing the same problem in to structs.

[HaloFour]

@eyalsk @DavidArno

I think we're wading into the territory of a different discussion here.

I just tested and answered my questions. C# does currently support consuming structs that define default constructors. It just doesn't support defining them. There is one notable exception and that's in a generic method where the type has a constructor constraint. I believe that this was already considered a bug in the JIT.