Support for WebAssembly Threads

Starting from .NET 7, experimental support for WebAssembly threads has been included. This support is provided by the boostrapper 7.0 and later, for interpreter and AOT modes. The following documentation explains how to enable threading.


Threading support is now supported in most major browsers. You can find out if your target browser supports it on the WebAssembly roadmap.

Enabling threads

Add the following to your WebAssembly project:


Threading support can be detected at runtime by using the UNO_BOOTSTRAP_MONO_RUNTIME_FEATURES environment variable.

Controlling the browser thread pool size

By default, the runtime pre-creates a set of 4 WebWorkers available to WebAssembly threads, and the .NET Runtime will reuse the workers as threads stop. This value is used by the ThreadPool and Tasks subsystems.

To change the number of threads available to the app, use the following:


The maximum number of threads can be determined by the UNO_BOOTSTRAP_MAX_THREADS environment variable.


WebAssembly Threads are using the SharedArrayBuffer browser feature, which is disabled in most cases for security reasons.

To enable SharedArrayBuffer, your website will need to provide the following headers:

Cross-Origin-Embedder-Policy: require-corp
Cross-Origin-Opener-Policy: same-origin

If you're using resources from another origin, that origin should also be serving the following header to accept being loaded by the app:

Cross-Origin-Resource-Policy: cross-origin

See here additional information about this header.

You can find more information about threading on the Chrome Developer blog.

If you're using dotnet serve or any other similar server package to serve your application, you can use the following:

dotnet serve -p 8000 -h "Cross-Origin-Embedder-Policy: require-corp" -h "Cross-Origin-Opener-Policy: same-origin"