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I have a simple Form with buton and one event button1_Click:

public Form1 : Form

 public Form1()
 
 ...
 this.button1.Click += new System.EventHandler(this.button1_Click);
 

 private async Task button1_Click(object sender, EventArgs e) // line A
 
 

In line A I would like to try use async Task return type rather than void and I encounter error message from compiler. How I can make this code run?

That event handler is a delegate with void return type, which means you can't add methods with Task result to it.

However you can also mark void methods with async so you should just replace the Task with void.

We can check this code:

 public partial class Form1 : Form

 public Form1()
 
 InitializeComponent();
 this.button1.Click += (s, e) => Button1_Click(s, e);
 //this.button1.Click += Button1_Click1;
 MessageBox.Show("The End!");
 
 private async Task Button1_Click(object sender, EventArgs e)
 
 await Task.Delay(5000);
 MessageBox.Show("With Sync");
 

 private void Button1_Click1(object sender, EventArgs e)
 
 MessageBox.Show("Without async");
 

How I can make this code run?

It cannot be done because the async Task signature is not implemented for UI event handlers in .net. (make sure to read this post: Why do event handlers always have a return type of void?)

The general consensus is that, in general, a return value of an event handler has not a well defined purpose. Although IMO it would make sense to be able to fire off all the handlers async, but then again, that would be exactly the case if you wouldn't await the logic inside it.

Of course you could overcome the problem by putting the event logic in another class. Most of the time this is actual a good idea anyhow.

Note: do not use it like this, it's just to explain it

public Form1 : Form

 public Form1()
 
 ...
 this.button1.Click += new System.EventHandler(this.button1_Click);
 

 private async void button1_Click(object sender, EventArgs e) // line A
 
 await YourLogic(e.foo); //optional configure await.
 

 private async Task YourLogic(your parameters)
 
 // do stuff
 

See also:

Why do event handlers always have a return type of void?

async/await - when to return a Task vs void?

https://msdn.microsoft.com/en-us/magazine/jj991977.aspx

How do I await events in C#?

How to make form events to use benefits from async?

So, basically it already does, but check out this post:
How to 'await' raising an EventHandler event