IDE0028 warns to use an unsupported preview feature, collection expression arguments

[anthonymastreanvae]

Version Used: .NET 10.0.103

I have LangVersion set to latest, but this analyzer is showing a warning and recommending a fix that breaks the build. Not sure why the initial warning doesn't fail the build, as I also have TreatWarningsAsErrors set to true on CI.

Dictionary<string, object> dict1 = [];
Dictionary<string, object> dict2 = new(dict1); // ⚠️ IDE0028: Collection initialization can be simplified

Diagnostic Id: IDE0028

Use collection initializers or expressions (IDE0028)

Expected Behavior:

I did not expect an analyzer to recommend unsupported preview features! 😁

Collection expression arguments

https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/collection-expressions#collection-expression-arguments

Actual Behavior:

Accept the codefix:

Dictionary<string, object> dict1 = [];
Dictionary<string, object> dict2 = [with(dict1)]; // ❌ CS8652: The feature 'collection expression arguments' is currently in Preview and *unsupported*. To use Preview features, use the 'preview' language version.

[MihuBot]

I'm a bot. Here is a possible related and/or duplicate issue (I may be wrong):

• C# suggests using collection expression arguments even though LangVersion is set to 14 and .NET 11 Preview 1 is not installed microsoft/vscode-dotnettools#2789

[anthonymastreanvae]

Neat, might already be fixed? Not sure what version will have the fix though.

#82389

[anthonymastreanvae]

Ah, activity on the PR suggests 10.0.3xx or 11.0.1xx.