Proposal: Declaration of anonymous type

[alexzhaosheng]

I really like the anonymous type in TypeScript, it is quite convenient when you are creating a type that is used just for one time.
Now the anonymous type of C# doesn't support declaration, this limits it's usage.
I would like to see something like this:

foo({int Value; string Description} arg){
    var xValue = arg.Value;
    var xDescription = arg.Description;

   //and maybe this?
  var areTheSameType = arg.GetType() == typeof({int Value; string Description}); //true
  areTheSameType = arg is {int Value; string Description}; //true
}


//call foo
foo(new {Value=123, Description="Test"}); //it works

foo(new {Value="str value", Description="Test"}); //compile error

[ufcpp]

This proposal may be the same as "denotable" anonymous type / Record in #98

[alexzhaosheng]

Hi ufcpp,

Thanks for your information.
I failed to find the details about this "denotable anonymous type" . But it sounds like the thing that I want. Can't wait to try it.

[HaloFour]

Exposing the type makes it no longer anonymous. Whatever that type is would have to be exposed with at least the same level of accessibility as the method itself and if for whatever reason the algorithm that has to generate the declared name for that type you would break all compiled consumers.

Also, since there is no type equivalence amongst anonymous types it wouldn't be possible to pass one from one assembly to another. Nor would typeof or is work since those facilities are provided by the CLR and according to the CLR they would be different types.

I think you'll find better utility with tuples (#347), since they'll all share the same underlying BCL structs.

[alexzhaosheng]

Hi HaloFour,

Thanks for replying.
I think "Tuple" you mentioned is a better way to deal with this situation, anonymous should not be take as a normal type.

[dsaf]

Related to #3304.

[gafter]

Records are named types that seem to satisfy this need. For structs you could use tuples.

[alrz]

Doesn't this have type equivalency problem between the assemblies?

[neurospeech]

Using anonymous types everywhere will lead to refactoring problem. You will eventually create so many classes that could easily fit in hierarchy with base classes with least number of properties.

[gafter]

We are now taking language feature discussion in other repositories:

• https://github.com/dotnet/csharplang for C# specific issues
• https://github.com/dotnet/vblang for VB-specific features
• https://github.com/dotnet/csharplang for features that affect both languages

Features that are under active design or development, or which are "championed" by someone on the language design team, have already been moved either as issues or as checked-in design documents. For example, the proposal in this repo "Proposal: Partial interface implementation a.k.a. Traits" (issue 16139 and a few other issues that request the same thing) are now tracked by the language team at issue 52 in https://github.com/dotnet/csharplang/issues, and there is a draft spec at https://github.com/dotnet/csharplang/blob/master/proposals/default-interface-methods.md and further discussion at issue 288 in https://github.com/dotnet/csharplang/issues. Prototyping of the compiler portion of language features is still tracked here; see, for example, https://github.com/dotnet/roslyn/tree/features/DefaultInterfaceImplementation and issue 17952.

In order to facilitate that transition, we have started closing language design discussions from the roslyn repo with a note briefly explaining why. When we are aware of an existing discussion for the feature already in the new repo, we are adding a link to that. But we're not adding new issues to the new repos for existing discussions in this repo that the language design team does not currently envision taking on. Our intent is to eventually close the language design issues in the Roslyn repo and encourage discussion in one of the new repos instead.

Our intent is not to shut down discussion on language design - you can still continue discussion on the closed issues if you want - but rather we would like to encourage people to move discussion to where we are more likely to be paying attention (the new repo), or to abandon discussions that are no longer of interest to you.

If you happen to notice that one of the closed issues has a relevant issue in the new repo, and we have not added a link to the new issue, we would appreciate you providing a link from the old to the new discussion. That way people who are still interested in the discussion can start paying attention to the new issue.

Also, we'd welcome any ideas you might have on how we could better manage the transition. Comments and discussion about closing and/or moving issues should be directed to #18002. Comments and discussion about this issue can take place here or on an issue in the relevant repo.

---

I am not moving this particular issue because I don't have confidence that the LDM would likely consider doing this. For nominal types, you may be interested in records (dotnet/csharplang#39). For structural types, you may be interesed in tuples (dotnet/csharplang#59).