What is async method in C#?

The async modifier is used on a method, lambda expression or an anonymous method to create asynchronous methods. An async method runs synchronously until it reaches its first await operator, at which point the method is suspended while the awaited task is completed. In the meantime, the control returns to the caller of the method.

The async keyword turns a method into an async method, which allows you to use the await keyword in its body. When the await keyword is applied, it suspends the calling method and yields control back to its caller until the awaited task is complete. await can only be used inside an async method.

 https://www.c-sharpcorner.com/article/async-and-await-in-c-sharp/

See Asynchronous programming in C# at:

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/

async is not a method, it is a modifier.

See the article Async And Await In C# at:

https://www.c-sharpcorner.com/article/async-and-await-in-c-sharp

That article calls async and await keywords. I think it is better to be clear about language items instead of calling everything keywords.

I am not totally familiar with async and await but one thing I am sure about. Neither async nor await create tasks; tasks must be created by something else. Read the Microsoft article, you will see that await causes the user (the main thread or some other thread) to wait. As you can see in the first example in that article, simply adding async and await does not make the program finish sooner.

Using a different operating system, I have been doing multitasking (multithreading) since before Windows existed. So I know how to do multithreading using two very different operating systems yet I have difficulty understanding async and await. I think one reason they are difficult to understand is that no one has been able to explain them clearly enough.

At first remeber two things

1. Async doesn't do anything.
2. await doesn't mean to wait for the result to come.

Then what does async mean?

Async just tells the compilar that if anywhere in the method if a variable is declared with await name then please throw error, because await has reserved meaning please use it as keyword.

The meaning is , async simply tells the compilar that await is going to be used beware of that , if it is used as variable name then throw warning..

What does await do if he doesn't wait?

await simply let the Main/UI thread free to do other things and does not wait for the long task to complete , there is a state machine involved which tells the compilar when task is completed.

C# asynchronous method is a special method that executes asynchronously. C# provides async modifier to make a method asynchronous. It is used to perform asynchronous tasks.

C# await expression is used to suspend the execution of a method.

If a method which uses async modifier does not contain await expression, executes synchronously.

An async method can use any one of the following return types.

• Task
• Task
• Void (for event handlers)
• System.Threading.Tasks.ValueTask

for more details please refer

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/concepts/async/ 
https://www.c-sharpcorner.com/article/async-and-await-in-c-sharp/