Async, Tasks, and Promises

C# Type	JS Type
Task	Promise<void>
Task<T>	Promise<T>

Async methods

A .NET method that returns a Task or Task<T> can be awaited in JavaScript because the Task is automatically marshalled as a JS Promise:

[JSExport]
public static class AsyncExample
{
    public async Task<string> GetResultAsync();
}

const result = await AsyncExample.getResultAsync();

A JavaScript method that returns a Promise can be awaited in C# by converting the Promise to a Task:

JSFunction exampleAsyncJSFunc = …
string result = await exampleAsyncJSFunc.CallAsStatic(arg).AsTask<string>();

JSPromise type

The JSPromise type supports working directly with JavaScript Promise values, while the extension methods in the JSPromiseExtensions class enable converting between JSPromise and Task values.

Async execution

See JS Threading and Async Continuations for more about coordinating asynchronous .NET and JavaScript execution.