Added AsReadOnly() extension to ISet<T>

[Mrxx99]

this adds the asReadOnly extension method to ISet
unfortunately CollectionExtensions does not find ReadOnlySet, I guess it either has to be moved to System.Private.CoreLib and/or type forwarders have to be added. But I don't know how to do that.

fixes #29387

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[dotnet-issue-labeler]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

[stephentoub]

I guess it either has to be moved to System.Private.CoreLib

Yes, the implementation of ReadOnlySet<T> would need to be moved down to corelib. From a reference assembly perspective, it's in System.Collections.dll along with CollectionExtensions, so it's only an implementation issue. You can just move the implementation down.

[Mrxx99]

You can just move the implementation down.

Unfortunately I still get the following error:

C:\Code\Other\dotnet\runtime\src\libraries\System.Private.CoreLib\src\System\Collections\Generic\CollectionExtensions.cs(76
,23): error CS0246: The type or namespace name 'ReadOnlySet<>' could not be found (are you missing a using directive or an
assembly reference?) [C:\Code\Other\dotnet\runtime\src\coreclr\nativeaot\System.Private.CoreLib\src\System.Private.CoreLib.
csproj]

[stephentoub]

Unfortunately I still get the following error:

This is editing the wrong System.Private.CoreLib project file. You instead want to add it to https://github.com/dotnet/runtime/blob/main/src/libraries/System.Private.CoreLib/src/System.Private.CoreLib.Shared.projitems.

CoreLib for coreclr, mono, and native aot is slightly different for each. Most of the source is the same, and that shared source is listed in the above projitems file. The file you edited is the one that contains just the additional source specific to coreclr.

[eiriktsarpalis]

@stephentoub @artl93 can we make an exception and slot this API into .NET 9? It's low risk and complements the newly introduced ReadOnlySet<T>.

[stephentoub]

Thanks

[GerardSmit]

Sorry if I'm misunderstanding this, but if the type is moved from System.Collections to System.Private.CoreLib and this is landing in .NET 10 instead of .NET 9 (or for libraries built for .NET 9.0 preview), shouldn't there be a TypeForward?