Experimental WASI SDK for .NET Core

Wasi.Sdk is an experimental package that can build .NET Core projects (including whole ASP.NET Core applications) into standalone WASI-compliant .wasm files. These can then be run in standard WASI environments or custom WASI-like hosts.

How to use: Console applications

dotnet new console -o MyFirstWasiApp
cd MyFirstWasiApp
dotnet add package Wasi.Sdk --prerelease
dotnet build

You'll see from the build output that this produces bin/Debug/net7.0/MyFirstWasiApp.wasm.

To run it,

• Ensure you've installed wasmtime and it's available on your system PATH
• Run your app via dotnet run or, if you're using Visual Studio, press Ctrl+F5

Alternatively you can invoke runners like wasmtime or wasmer manually on the command line. For example,

• For wasmtime, run wasmtime bin/Debug/net7.0/MyFirstWasiApp.wasm
• For wasmer, run wasmer bin/Debug/net7.0/MyFirstWasiApp.wasm

Other WASI hosts work similarly.

How to use: ASP.NET Core applications

dotnet new web -o MyWebApp
cd MyWebApp
dotnet add package Wasi.Sdk --prerelease
dotnet add package Wasi.AspNetCore.Server.Native --prerelease

Then:

• Open your new project in an IDE such as Visual Studio or VS Code

• Open Program.cs and change the line var builder = WebApplication.CreateBuilder(args) to look like this:

  var builder = WebApplication.CreateBuilder(args).UseWasiConnectionListener();

• Open Properties/launchSettings.json and edit the applicationUrl value to contain only a single HTTP listener, e.g.,

  "applicationUrl": "http://localhost:8080"

• Open your .csproj file (e.g., in VS, double-click on the project name) and, inside a <PropertyGroup>, add this:

  <WasiRunnerArgs>--tcplisten localhost:8080 --env ASPNETCORE_URLS=http://localhost:8080</WasiRunnerArgs>

  Instead of 8080, you should enter the port number found in Properties\launchSettings.json.

That's it! You can now run it via dotnet run (or in VS, use Ctrl+F5)

Optionally, to add support for bundling wwwroot files into the .wasm file and serving them:

• Add the NuGet package Wasi.AspNetCore.BundledFiles

• In Program.cs, replace app.UseStaticFiles(); with app.UseBundledStaticFiles();

• In your .csproj file, add:

  <ItemGroup>
      <WasmBundleFiles Include="wwwroot\**" />
  </ItemGroup>

What's in this repo

• Wasi.Sdk - a package that causes your build to produce a WASI-compliant .wasm file. This works by:
  • Downloading the WASI SDK, if you don't already have it
  • When your regular .NET build is done, it takes the resulting assemblies, plus the .NET runtime precompiled to WebAssembly, and uses WASI SDK to bundle them into a single .wasm file. You can optionally include other native sources such as .c files in the compilation.
• Wasi.AspNetCore.BundledFiles - provides UseBundledStaticFiles, and alternative to UseStaticFiles, that serves static files bundled into your .wasm file. This allows you to have single-file deployment even if you have files under wwwroot or elsewhere.
• Wasi.AspNetCore.Server.Native - a way of running ASP.NET Core on WASI's TCP-level standard networking APIs (e.g., sock_accept). These standards are quite recent and are currently only supported in Wasmtime, not other WASI hosts.

... and more

Building this repo from source

First, build the runtime. This can take quite a long time.

• git submodule update --init --recursive
• Do the following steps using Linux or WSL:
  • sudo apt-get install build-essential cmake ninja-build python python3 zlib1g-dev
• cd modules/runtime/src/mono/wasm
  • make provision-wasm (takes about 2 minutes)
  • make build-all (takes 10-15 minutes)
    • If you get an error about setlocale: LC_ALL: cannot change locale then run sudo apt install language-pack-en. This only happens on very bare-bones machines.
• cd ../wasi
  • make (takes a few minutes - there are lots of warnings like "System is unknown to cmake" and that's OK)

Now you can build the packages and samples in this repo:

• Prerequisites
  • .NET 7 (dotnet --version should return 7.0.100-preview.4 or later)
  • Rust and the wasm32-unknown-unknown target (technically this is only needed for the CustomHost package)
    • Install Rust
    • rustup target add wasm32-unknown-unknown
• Just use dotnet build or dotnet run on any of the samples or src projects, or open the solution in VS and Ctrl+F5 on any of the sample projects