Async and Await in C

async and await are used when we are doing asynchronous programming. Why we would want to do asynchronous programming, is due to performance issues. When we have two unrelated tasks that are in the program, and one task takes a long time to process, it should not be holding up the other task.

We use Asynchronous programming to hand over program controls to ensure that no one process is holding up the entire program.

async and await are used together to make a program asynchronous.

Below is a simple example to make a program asynchronous

static void Main(string[] args)
{
    DoSomething();
    Console.WriteLine("Control to Main!");
    Console.ReadLine();
}

public static async void DoSomething()
{
    await Delay();
    Console.WriteLine("Control back to Method!");
}

async static Task Delay()
{
    await Task.Delay(5000);
}

The output on the console:
Control to Main!
<after a 5 second delay>
Control back to Method!

You first have to declare the method asynchronous with async
async static Task Delay()
Next, we put the keyword await beside the command that will take a long time.
What await does is this

– It awaits for the command to be completed

– While it is awaiting, it passes control back up to the caller

– After the command is completed, the control is passed back to the callee
If we look at the code, the program first calls DoSomething(), which calls await Delay(), which then executes await Task.Delay(5000);
When await Task.Delay(5000); is executed, Delay() passes control back up to the caller, which is DoSomething()
Because DoSomething() is awaiting Delay(), it passes control back up again to Main(), which then executes Console.WriteLine("Control to Main!");
After await Task.Delay(5000); is completed, it returns to DoSomething(), which executes Console.WriteLine("Control back to Method!");, and finally returns to Main()
Another example is given below
async Task<int> AccessTheWebAsync()
{
    HttpClient client = new HttpClient();

   Task<string> getStringTask = client.GetStringAsync("http://msdn.microsoft.com");

   // You can do work here that doesn't rely on the string from GetStringAsync.
   DoIndependentWork();

   string urlContents = await getStringTask;
   //The thing is that this returns an int to a method that has a return type of Task<int>
   return urlContents.Length;
}

Here, Task getStringTask = client.GetStringAsync("http://msdn.microsoft.com"); is called, followed by DoIndependentWork();.
We next call string urlContents = await getStringTask;, which awaits on getStringTask. There are two possible scenerios here

After DoIndependentWork() is completed, Task getStringTask = client.GetStringAsync("http://msdn.microsoft.com"); is completed as well, and getStringTask is fully initialized. In this case, there is no control being passed back to the caller of AccessTheWebAsync(), and the program just runs through.
After DoIndependentWork() is completed, Task getStringTask = client.GetStringAsync("http://msdn.microsoft.com"); NOT completed, and getStringTask is NOT initialized. In this case, there control is passed back to the caller of AccessTheWebAsync() for execution. Only after getStringTask has been initialized, will the program pass back control to AccessTheWebAsync()

And for obvious reasons, async cannot be a modifier on the Main method, because it is the root caller.
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