VSTHRD106.md

VSTHRD106 Use InvokeAsync to raise async events

Asynchronous events (those typed as AsyncEventHandler) must be raised carefully to ensure all event handlers are invoked and awaited on.

Although C# lets you invoke event handlers naturally, it has no awareness of async event handlers and thus will not let you correctly await on their invocation nor invoke them sequentially.

Examples of patterns that are flagged by this analyzer

public AsyncEventHandler Clicked;

async Task OnClicked() {
    await Clicked(this, EventArgs.Empty); // only awaits the first event handler.
}

Solution

Use the InvokeAsync extension method defined in the TplExtensions class and await its result. This will ensure each event handler completes before invoking the next event handler in the list, similar to the default behavior for raising synchronous events.

public AsyncEventHandler Clicked;

async Task OnClicked() {
    await Clicked.InvokeAsync(this, EventArgs.Empty); // await for the completion of all handlers.
}