Need C# JSImport async signature interop example to match async Javascript method - Blazor

Question

In Blazor, Core 7, I don't know how to create a JSImport signature to match an async javascript method. Here's an example of the Javascript method:

export async function DoSomething(data)
{
    // do work
    return result
}

the following C# signature doesn't compile because of the async and Task

    // assume ImportAsync has already been called
    [JSImport("DoSomething", "MyModule/DoSomething.js")]
    internal static async partial Task<byte[]> DoSomething(byte[] data);

The microsoft literature was no help and there's no async examples here: https://github.com/pavelsavara/dotnet-wasm-todo-mvc

Am I supposed to use a callback? That would seem rather primitive and destroys the async model

Answer 1

The import

[JSImport("getMessage", "SampleJS")]
[return: JSMarshalAs<JSType.Promise<JSType.Any>>()]
internal static partial Task<object> 
  GetWelcomeMessage([JSMarshalAs<JSType.Array<JSType.Number>>] byte[] bytes);

JS module

export async function getMessage(data) {
  console.log(data)
  return await new Promise((resolve) => {
    setTimeout(() => {
      resolve(data);
    }, 2000);
  });
}

This just logs the data (bytes) received, waits 2s and returns the same data.

Usage

byte[] sampleData = Encoding.UTF8.GetBytes("Hello from C#");
object? result = await GetWelcomeMessage(sampleData);
if (result is byte[] bytes)
{
    message = Encoding.UTF8.GetString(bytes);
    Console.WriteLine($"Got {message} from {result.GetType()}");
}