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by Jiří {x2} Činčura

Tasks - try on retrying, loops, recursions

22 Mar 2013 .NET, C#, Multithreading/Parallelism/Asynchronous/Concurrency

My mind today was in weird shape (again). I blame Friday. What that means is, that I came with weird pieces of code, that are fun to just write. Brain training. :)

The set up was simple. I had a Task that was returning bool; true if succeeded, false otherwise. Because the code was interacting with 3rd party system it was desired to retry if the call didn’t succeeded.

Looking at it in front of me I realized I’ll be probably able to (ab)use the ContinueWith method and recursion. When I wrote it I realized there’s a room for refactoring. And then I realized I might create and extension method from it. Already half way in hell. :)

public static Task<TResult> Retry<TResult>(this Func<Task<TResult>> taskMethod, Func<TResult, bool> resultOK, int retries)
	return taskMethod()
		.ContinueWith(t =>
			retries == 0 || resultOK(t.Result)
				? Task.FromResult(t.Result)
				: Retry(taskMethod, resultOK, --retries))

Umm, how it will look like with await, I thought. Then I can use loop.

public static async Task<TResult> Retry2<TResult>(this Func<Task<TResult>> taskMethod, Func<TResult, bool> resultOK, int retries)
	while (true)
		var result = await taskMethod();
		if (retries-- == 0 || resultOK(result))
			return result;

This doesn’t look nice. I thought it will be smoother. This is like 90s. Back to recursion.

public static async Task<TResult> Retry3<TResult>(this Func<Task<TResult>> taskMethod, Func<TResult, bool> resultOK, int retries)
	var result = await taskMethod();
	return retries == 0 || resultOK(result)
		? result
		: await Retry3(taskMethod, resultOK, --retries);

That looks better. I don’t know whether I like the first or the third version more. But. What about performance? The second version has least code to execute. No doubt. The third version has two awaits and the recursion (nesting) with awaits. This is probably not going to be good for performance. And my tests confirms that. Slightly slower than second is the first version. Thus the winner for me is version #1. 8-)

I’m not sure why I did this (from practical perspective), but it was fun. Have a great Friday.