How do I define a Func where it may be async

I am writing a method where it will get a parameter like this:

ShowMessagePopupAsync(string header, string message, Func<string, Task> onClick, Func<Task>? onClose)

First off, onClose may be null hence the ?.

But how should I declare the return value of the Func? It could be an async method where the code on the OnClick needs to use await and return a Task. Or it may be as simple as saving the passed string and setting a bool.

What should my parameter be to handle both of these?

public void OnClick(string message) {
    savedMessage = message;
}
    
public async Task<void> OnClick (string message) {
    var x = await SaveTheInfo (message);
    if (x > 3)
        await CallSomeone();
}

You can change the first method to look like this:

public Task OnClick(string message) {
    savedMessage = message;
    return Task.CompletedTask;
}

This first one doesn't have anything that needs to execute asynchronously, so you can just skip the async keyword and return Task.CompletedTask, which is useful for scenarios like this.

public async Task OnClick(string message) {
    var x = await SaveTheInfo(message);
    if (x > 3)
        await CallSomeone();
}

This second one can stay the way it is. You just don't need the Task<void> in the signature, because it's essentially the same as Task.

When two methods share the exact same signature, they cannot exist in the same class. In this case, both methods do share the same signature, the async keyword does not make a difference, because the compiler removes it.