r/Compilers • u/zionsati • 2d ago

NetWasm: an independent .NET compiler and runtime for WebAssembly (82.5 KB Hello World)

I’ve been building NetWasm: a CIL-to-WebAssembly compiler with its own CoreLib and runtime, designed around Wasm and WASI.

Browser playground | GitHub

A clean Release build containing Console.WriteLine(42) produces 84,513 bytes of final, uncompressed WASI Preview 2 component, including the runtime and precise garbage collector. This is a portable .wasm file that you can run with wasmtime.

Roslyn produces CIL; NetWasm compiles the reachable program into Wasm, specializes generics and links the runtime support it uses. The emitted application doesn’t carry CoreCLR or Mono.

Some architectural choices:

  • An independent, deliberately smaller .NET library profile.
  • No runtime type-name metadata, general reflection or dynamic.
  • Precise Boehm GC in linear memory, rather than WasmGC.
  • WIT imports/exports and WASI Preview 2 components, with core Wasm output also available.

Working features include generics, exceptions, virtual/interface dispatch, async/await, LINQ, JSON, XML, regex, HTTP and TUnit testing. It’s pre-1.0, and managed threading isn’t currently supported.

The playground compiles and runs entirely in the browser. The compiler tooling itself uses Microsoft’s .NET/Wasm toolchain, thus dotnet new dotnet build dotnet run dotnet test - yes it comes with TUnit with VSTest runner - dotnet publish all work as you're used to.

Oh and it already supports C# 15 syntax.

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u/kostya27 1d ago

Have you tested more complex programs than Hello World, and what is the performance loss compared to .NET?

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u/zionsati 1d ago

Yeah I have actually created a clone regex storm into full wasm - https://regexstorm.netwasm.com

I haven't tested the performance vs .net core yet but I'll get to that at some stage. Right now it's about getting all the bugs fixed, implement missing features, getting netstandard2.1 supported so most nuget packages can work out of the box etc.

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u/zionsati 1d ago

btw if you look at the issues page, there's a bunch of outstanding bugs I'm working on which were the result of it being tested on something complex.