Using async/await with IAsyncResult pattern

Even though in my current project we use .NET 4.5 we still have lots of references to internal libraries targeting .NET 4.0. Usually it is not a problem, however some of these libraries follow Asynchronous Programming Model pattern. This approach makes the code quite ugly because we end up with lots of nested callbacks.

1
// rest of the code omitted for brevity
2
private static void GetHostEntryIAsyncResultPattern()
3
{
4
    DnsService dnsService = new DnsService();
5
    dnsService.BeginGetHostEntry(GoogleHost, googleAsyncResult =>
6
    {
7
        var googleHostResult = dnsService.EndGetHostEntry(googleAsyncResult);
8
        Console.WriteLine($"Result from IAsyncResultPattern: {GoogleHost} - {FormatHostEntry(googleHostResult)}");
9
        dnsService.BeginGetHostEntry(YahooHost, yahooAsyncResult =>
10
        {
11
            var yahooHostResult = dnsService.EndGetHostEntry(yahooAsyncResult);
12
            Console.WriteLine($"Result from IAsyncResultPattern: {YahooHost} - {FormatHostEntry(yahooHostResult)}");
13
        }, null);
14
    }, null);
15
}
16
// rest of the code omitted for brevity

Fortunately thanks to Task.Factory.FromAsync method it is possible to quite easily make Begin/End methods work with async/await pattern. All you have to do is to pass Begin/End methods as an arguments to Task.Factory.FromAsync and you are good to go. The original code after refactoring looks like that

1
// rest of the code omitted for brevity
2
var dnsService = new DnsService();
3
var googleHostResult = await Task.Factory.FromAsync((callback, stateObject) => dnsService.BeginGetHostEntry(GoogleHost, callback, stateObject), dnsService.EndGetHostEntry, null);
4
Console.WriteLine($"Result from async/await: {GoogleHost} - {FormatHostEntry(googleHostResult)}");
5

6
var yahooHostResult = await Task.Factory.FromAsync((callback, stateObject) => dnsService.BeginGetHostEntry(YahooHost, callback, stateObject), dnsService.EndGetHostEntry, null);
7
Console.WriteLine($"Result from async/await: {YahooHost} - {FormatHostEntry(yahooHostResult)}");
8
// rest of the code omitted for brevity

I think everyone will agree with me that refactored version is much better than original one.

Source code for this post can be found here