James Montemagno
James Montemagno

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C# Developers: Stop Calling .Result

James MontemagnoJames Montemagno

I love async/await Task based programming! I talk about it in all my demos, it is in all my libraries, and Frank and I did a whole Merge Conflict podcast episode on it. The problem is that I keep getting bug reports and seeing samples where it is being abused and used completely wrong. I am never a fan of calling a lot of async code in a constructor or in the OnCreate/AppDelegate, but the real issue is when I see stuff like this with .Result:

public App()
{
    try
    {
        _locator.StartListeningAsync(TimeSpan.FromSeconds(30), 100);
        var position = _locator.GetPositionAsync().Result;
        if (position != null)
        {
            App.lat = position.Latitude;
            App.lon = position.Longitude;
        }
    }
    catch (Exception c)
    {

    }

    MainPage = GetMainPage();
}

AHHHHHH!!! We aren't letting StartListeningAsync finish, and we are doing a .Result essentially locking the entire app start code! This makes me the saddest panda in the entire world.

Recently Jon Goldberger wrote a great article on getting started with async/await, but going way back 6 years ago, Stephen Toub from Microsoft 6 wrote an amazing article explaining how to use async/await when dealing with the UI and with event handlers. Both article are great reads and yes even the 6 year old blog applies to this day.

Stephen's article boils down to this:

On your UI thread, instead of writing:

Task s = LoadStringAsync(); 
textBox1.Text = s.Result; // BAD ON UI

you can write:

Task s = LoadStringAsync(); 
textBox1.Text = await s; // GOOD ON UI

Or instead of writing:

Task t = DoWork(); 
t.Wait(); // BAD ON UI

you can write:

Task t = DoWork(); 
await t; // GOOD ON UI

Essentially calling .Result or .Wait will lock up your UI! It is true! So true that I made an entire video right here explaining why you need to stop using .Result!

Live, Love, Bike, and Code

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