Uno.Wasm.Bootstrap provides a simple way to package C# .NET code, and run it from a compatible browser environment.

It is a standalone .NET Web Assembly (Wasm) sdk bootstrapper taking the form of a nuget package.

Installing it on a .NET project (5, 6 or .NET Standard 2.0) with an entry point allows to publish it as part of a Wasm distribution folder, along with CSS, Javascript and content files.

This package only provides the bootstrapping features to run a .NET assembly and write to the javascript console, through Console.WriteLine.

This package is based on the excellent work from @praeclarum's OOui Wasm MSBuild task.

How to use the bootstrapper with .NET 5 and later

  • Create a .NET 5 Console Application, and update it with the following basic definition:
<Project Sdk="Microsoft.NET.Sdk.Web">

    <MonoRuntimeDebuggerEnabled Condition="'$(Configuration)'=='Debug'">true</MonoRuntimeDebuggerEnabled>

    <PackageReference Include="Uno.Wasm.Bootstrap" Version="2.1.0" />
    <PackageReference Include="Uno.Wasm.Bootstrap.DevServer" Version="2.1.0" PrivateAssets="all" />

  • Add a main entry point:
class Program
    static void Main(string[] args)
        Console.WriteLine("Hello from C#!");
  • In Visual Studio 2019, press Ctrl+F5 to start without the debugger (this will create the launchSettings.json needed below for debugging)
  • A browser window will appear with your application
  • The output of the Console.WriteLine will appear in the javascript debugging console

How to use the Visual Studio 2019/2022 Debugger

Starting from Visual Studio 2019 16.6, it is possible to debug a WebAssembly app.

To enable the debugging, add the following line to your launchSettings.json file:

"inspectUri": "{wsProtocol}://{url.hostname}:{url.port}/_framework/debug/ws-proxy?browser={browserInspectUri}"

in every profile section of the file, below each "launchBrowser": true, line.

Press F5 to start debugging.

Alternate deployment methods

Install the dotnet serve tool:

dotnet tool install -g dotnet-serve

Once installed, launch the server by using the following command:

cd MyApp.Wasm
dotnet serve -d bin\Debug\net5.0\dist -p 8000

You application will be available http://localhost:8000.

Upgrading from previous versions of the Uno.Wasm.Bootstrap package

Previously, the suggested project structure was a .NET Standard 2.0 project using the non-web projects SDK. To enable debugging and easier deployment, the support for Microsoft.NET.Sdk.Web has been added.

To upgrade a project from 1.1 to 1.2:

  • If you had a <DotNetCliToolReference /> line, remove it
  • Add the <PackageReference Include="Uno.Wasm.Bootstrap.DevServer" Version="1.2.0-dev.1" PrivateAssets="all" /> item in the same item group as the other nuget packages.

To upgrade a project from 1.0 to 1.1:

  • Change Microsoft.NET.Sdk to Microsoft.NET.Sdk.Web in the Sdk attribute of your project
  • Add the <DotNetCliToolReference Include="Uno.Wasm.Bootstrap.Cli" Version="1.0.0-dev.1" /> item in the same item group as the other nuget packages.

Changing the .NET SDKs install location

The SDKs are installed under Path.GetTempPath() by default, you may change this by setting the following msbuild property(or environment variable): WasmShellMonoTempFolder.

For example, on Windows, setting WasmShellMonoTempFolder to C:\MonoWasmSDKs, the mono-wasm-e351637985e sdk would be installed under C:\MonoWasmSDKs\mono-wasm-e351637985e

Bootstrapper versions and .NET runtimes

Each major version of the bootstrapper targets a different version of the .NET Runtime.


Starting from version 3.x, the bootstrapper uses a custom build of the runtime, maintained here:


Bootstrapper builds version 4.x-dev were based on developments builds of .NET 7 and were later versioned 7.x-dev to match the appropriate runtime.