c# - How Async and Await works

Question

I am trying to understand how Async and Await works. How control travel in the program. Here is the code which I was trying to understand.

public async Task MyMethod()
{
    Task<int> longRunningTask = LongRunningOperation();
    //indeed you can do independent to the int result work here 
    MySynchronousMethod();
    //and now we call await on the task 
    int result = await longRunningTask;
    //use the result 
    Console.WriteLine(result);
}

public async Task<int> LongRunningOperation() // assume we return an int from this long running operation 
{
    await Task.Delay(5000); //5 seconds delay
    return 1;
}

private void Button_Click_3(object sender, RoutedEventArgs e)
{
     MyMethod();
}

When button click occur then MyMethod() will be called and from the MyMethod LongRunningOperation() will be called and it take 5 sec to complete. so my question is

• What is the meaning of this line

  Task longRunningTask = LongRunningOperation();

• what this does int result = await longRunningTask;

The above line could be committed and one line we can construct like

Task<int> longRunningTask = await LongRunningOperation();

or

int result = await longRunningTask;

Please can someone explain to me the above code which is not clear to me.

1) if the longRunningOperation hasn't finished and is still running, MyMethod() will return to its calling method, thus the main thread doesn't get blocked. When the longRunningOperation is done then a thread from the ThreadPool (can be any thread) will return to MyMethod() at its previous state and continue execution (in this case printing the result to the console).

A second case would be that the longRunningOperation has already finished its execution and the result is available. When reaching the await longRunningOperation the compiler knows that it has the result and will keep on executing code on the very same thread. (in this case printing result to console).

point 1 is not at all clear to me like the statement "if the longRunningOperation hasn't finished and is still running, MyMethod() will return to its calling method"

if possible explain the point one in more detail. thanks

Answer 1

I've been taught about it in the following fashion, I found it to be quite a clear and concise explanation:

//this is pseudocode
async Method()
{
    code;
    code;
    await something; 
    moreCode;
}

When Method is invoked, it executes its contents (code; lines) up to await something;. At that point, something; is fired and the method ends like a return; was there.

something; does what it needs to and then returns.

When something; returns, execution gets back to Method and proceeds from the await onward, executing moreCode;

In a even more schematic fashion, here's what happens:

1. Method is invoked
2. code; is executed
3. something; is executed, flow goes back to the point where Method was invoked
4. execution goes on with what comes after the Method invocation
5. when something; returns, flow returns inside Method
6. moreCode; is executed and Method itself ends (yes, there could be something else await-ing on it too, and so on and so forth)