It has not been easy to follow progress on WASI support in .NET 9.
Since Rich Lander's blog post, shortly after the .NET 8 release, there has been no news about WASI from Microsoft.
Tracking the progress on Github is not easy either as there are many issues related to Wasm and WASI with these two seeming like the most important:
Early in the year it looked like WASI support would see some progress in .NET 9 with the above issues having 9.0.0 as Milestone.
One thing that indicated progress was the ability to build WASI as a library, see this issue and matching PR.
But then from early July the Milestones started changing to 10.0.0 on the issues concerning WASI support.
And by the time .NET Preview 7 was released it looked like .NET 9 would only support WASI Preview 2 and WASI Preview 1 support would be dropped because the team lacked the resources to maintain both, which seems fair. See this Github issue for context.
The latest change seems to be that there will be no support for wasi-experimental
in .NET 9 if this PR is any
indication. But it also looks like wasi-experimental will be back in .NET 10.
So to use .NET + WASI Preview 1 it seems you'll have to stay on .NET 8.
But if you would like some WASI (Preview 2) in .NET 9 things might not be all bad.
The BytecodeAlliance has released a Nuget package componentize-dotnet, see their
blog post.
This enables WASI Preview 2 and WASI components in .NET 9.
The blog post uses .NET Preview 7, but I also got the samples working with
.NET 9 RC 2. What works after .NET 9 RTM remains to be seen.