Pass args to C# WASM on startup

Profile pictureToni Petrina
Published on 2023-11-121 min read
  • #dotnet
  • #wasm
  • #blazor

I needed to get the host URL from WASM/Blazor class library - the simplest option seemed to pass the args on startup. However, that is not supported.

Blazor class library comes with the following initialization method:

private static async Task Main(string[] args)

And in js we initialize Blazor with the following (autostart`` is set to false`):

await Blazor.start("args", "are", "ignored");

While it would be awesome to just pass args directly, it is not possible. Luckily, a workaround is available with Blazor calling a special init function that we can expose. More info at GitHub issue.

Final code is:

  window.startupParams = function () {
    return [window.location.toString()];
<script src="_framework/blazor.webassembly.js" autostart="false"></script>

Finally, C# code is adjusted to read it in a very crude way:

var startupParams = js.Invoke<string[]>("startupParams");

var hostUrl = new Uri(startupParams?.Length > 0
    ? startupParams[0]
    : "http://localhost:3000");

Change code theme: