Update C# extension runtime to .NET 10

[claudiaregio]

[rkttu]

Hi, I'd like to share a real-world impact of this issue.

When opening a simple File-based Program (single .cs file) with .NET 10 SDK (GA), the language server takes ~40 seconds to load due to the runtime mismatch.

From the logs:

[info] Locating .NET runtime version 9.0.1
...
[info] .NET BuildHost started from ... reloading to start from C:\Program Files\dotnet\dotnet.exe to match necessary SDK location.
...
[info] 00:00:39.7479490에 있는 모든 프로젝트의 (재)로드가 완료됨

The extension starts with .NET 9 runtime, then the BuildHost restarts to match the .NET 10 SDK location, causing significant delay.

Since .NET 10 has been GA for about 2 weeks now, it would be great if this could be prioritized. The current behavior makes File-based Programs practically unusable for quick prototyping scenarios.

Environment:

• C# Extension: 2.100.11
• .NET SDK: 10.0.100
• OS: Windows

Thank you!

P.S. I've also shared this issue with the local .NET developer community in South Korea: https://forum.dotnetdev.kr/t/net-10-vs-code/14094

[JoeRobich]

To avoid the reload you could also install a .NET 9 SDK or Runtime alongside your .NET 10 SDK.

[rkttu]

@JoeRobich Thanks for the suggestion, but I tested installing .NET 9 SDK/Runtime alongside .NET 10, and unfortunately it didn’t improve the situation at all. The language server still takes ~40 seconds to load.

Even if this workaround were effective, I don’t think it’s a reasonable solution for the following reasons:

1. .NET 10 is GA - It’s not a preview or RC. It’s the official release, and users have a reasonable expectation that Microsoft’s own tooling supports it properly.

2. Microsoft removed .NET 8/9 download links from the official page - The official .NET download page no longer provides direct links to .NET 8 or 9. Asking users to hunt down older SDKs contradicts Microsoft’s own messaging to move forward with .NET 10.

3. I’m an educator creating course materials - File-based Programs are perfect for teaching scenarios, which is exactly why I’m using them. I cannot tell my students to “install an older SDK that’s no longer prominently available on the official download page” as a prerequisite. That’s confusing and undermines confidence in the platform.

4. Irony of the situation - File-based Programs are a new .NET 10 feature designed to lower the barrier for beginners and quick prototyping. But the tooling that should support this feature doesn’t work properly with .NET 10, defeating its purpose.

I respectfully request that this issue be prioritized. The current state makes File-based Programs practically unusable for real-world educational and prototyping scenarios.

[JoeRobich]

@rkttu Sorry it did not improve your scenario. If you believe there is a performance issue you can open an issue against https://github.com/dotnet/vscode-csharp/issues and follow these steps to get a performance trace.

[JoeRobich]

users have a reasonable expectation that Microsoft’s own tooling supports it properly.

We currently support .NET 10 development with the C# extension. Our language server does not have to be running on the .NET 10 runtime in order to properly load and provide features services for your .NET 10 projects. The BuildHost which loads your project does need to run on the .NET 10 runtime and that is why you see it relaunching in your logs.

Asking users to hunt down older SDKs ... cannot tell my students to “install an older SDK that’s no longer prominently available on the official download page” as a prerequisite.

It is unnecessary to install any other SDK than .NET 10 to support your File-based programs. Users will have to enable file-based program support in VS Code settings until we are ready to enable it by default.

the tooling that should support this feature doesn’t work properly with .NET 10, defeating its purpose.

If you are running into issues developing with file-based apps, please open an issue against https://github.com/dotnet/vscode-csharp/issues providing the necessary logs and repro steps.

I would also encourage you to try the prerelease version of the extension as it includes additional bug fixes and improvements.