How to create a generic extension method for async methods?

Question

I am trying to create a .WithDelay(seconds); method which I can add at the end of async method calls.

The issue I get is the async method is called first then the delay happens, I want it the other way around, without switching the order of the call.

For example, I want await MyMethod().WithDelay(seconds); rather than await WithDelay(seconds).MyMethod();.

Here is what I have so far, it calls the method first:

public async static Task<T> WithDelay<T>(this Task<T> task, int delay) {
  await Task.Delay(delay);
  return await task;
}

I expect the delay to happen first, then the actual method to run.

Answer 1

I want it the other way around, without switching the order of the call.

This isn't possible, because the C# language only supports extension methods on types, not methods.

The closest you can get is an extension method on delegates:

public static async Task<T> WithDelay<T>(this Func<Task<T>> func, int delay) {
  await Task.Delay(delay);
  return await func();
}

The usage still ends up being awkward:

// Either:
Func<Task<MyType>> func = MyMethod;
var result = await func.WithDelay(1000);

// or (assuming "using static"):
var result = await WithDelay(MyMethod, 1000);

// What you really want, not currently possible:
// var result = await MyMethod.WithDelay(1000);

For type-related situations like this, it can help to solve the problem synchronously first, then convert that solution to async. If the language prevents a good synchronous solution, then it will likely prevent a good asynchronous solution.

There is a proposal for extension methods on methods, but it's not part of the language today.