async/await does not work properly

Question

I have problem with async/await method of mine. I am new into async/await and seems I can't get it to work correctly.

The GUI freezes when I have the following.

private async void SetUpTextBox(string s)
{
    //This method is called in button event
    string textToSet = await GetTextA(s);
    read_Box.Text = textToSet;
}

private Task<string> GetTextA(string s)
{
    return Task.Run(() => GetText(s));
}

private string GetText(string s)
{
    string textToReturn = "Hello";
    using (StreamReader sr = new StreamReader(File.OpenRead(s)))
    {
        textToReturn = sr.ReadToEnd();
    }
    return textToReturn;
}

I don't know what I am doing wrong. I know I am new into this and that's why I am here to learn (a.k.a don't judge!).

P.S When I tried to change

using (StreamReader sr = new StreamReader(File.OpenRead(s)))
{
    textToReturn = sr.ReadToEnd();
}

With simple Thread.Sleep(2500) method. The GUI doesn't freeze at all!

Answer 1

Create an async void (for exemple Wait) with Thread.Sleep(x) inside and call it with "await Wait(x) ;" inside an async void.

Answer 2

Avoid async void except for async event handlers, plus you can use ReadToEndAsync and make code async all the way through.

private async Task SetUpTextBox(string s) {
    //This method is called in button event
    string textToSet = await GetTextAsync(s);
    read_Box.Text = textToSet;
}

private async Task<string> GetTextAsync(string s) {
    string textToReturn = "Hello";
    using (StreamReader sr = new StreamReader(File.OpenRead(s))) {
        textToReturn = await sr.ReadToEndAsync();
    }
    return textToReturn;
}

An example of calling SetUpTextBox within the button click event handler, as implied by the comment from original post

public async void OnBtnClicked(object sender, EventArgs args) {
    try {
        //...

        await SetUpTextBox("some path");

        //..
    } catch {
        //... handle error
    }
}

You should also put a try/catch around everything with the an async void event handler, otherwise any exceptions will go unobserved.

Reference Async/Await - Best Practices in Asynchronous Programming

Answer 3

First off, the things you should do, that you aren't:

• Using ReadToEndAsync.
• Avoiding Task.Run when the task is not CPU bound.
• Avoiding async void when the method is not an event handler.
• Having proper error handling.

Nkosi has done a decent job illustrating that.

What is left is explaining why Thread.Sleep(2500) does not block the UI.

---

First off, notice that Task.Run will be running the task using the ThreadPool.

You can verify that System.Threading.Thread.CurrentThread.IsThreadPoolThread is true inside of GetText (for example, you can set a break point and use inspections to check).

That means that the UI thread cannot be blocked inside of GetText, because the UI thread does not run GetText.

---

But why then Thread.Sleep(2500); yields a different result?

Hmmm... what is the difference between:

private string GetText(string s)
{
    string textToReturn = "Hello";
    using (StreamReader sr = new StreamReader(File.OpenRead(s)))
    {
        textToReturn = sr.ReadToEnd();
    }
    return textToReturn;
}

And:

private string GetText(string s)
{
    string textToReturn = "Hello";
    Thread.Sleep(2500);
    return textToReturn;
}

Let me tell you:

• The possible exceptions thrown. Since you are not handling them - and I assume you are using a debug build in a debugger - you would have noticed. So, I do not think this is the problem. Even if you weren't, the exceptions are not from the UI thread, would go unhandled, and crash the application (Edit: I tested).
• The ammount of time it takes to execute. However, since the UI will not be blocked by this code - being it running in the ThreadPool - that is not it either.
• The ammount of text returned.

When you have eliminated the impossible, whatever remains, however improbable, must be the truth

-- Sherlock Holmes

The UI is being stuck because you are returning too much text.

---

Let me put this way...

TOO MUCH TEXT RETURNED

private string GetText(string s)
{
    string textToReturn = "Hello";
    using (StreamReader sr = new StreamReader(File.OpenRead(s)))
    {
        textToReturn = sr.ReadToEnd();
    }
    return textToReturn;
}

A little text returned:

private string GetText(string s)
{
    string textToReturn = "Hello";
    Thread.Sleep(2500);
    return textToReturn;
}

---

The UI, is therefore being stuck at read_Box.Text = textToSet;, busy trying to render all the text that you got from the file. Or at least that is my hyphothesis.