Does marking a method async when it contains only synchronous calls actually causes async execution?

I have following method:

public async Task M1()
{
    for (int i = 0; i < 10; i++)
    {
        await M2();
    }
}

public async Task M2()
{
    for(int i = 0; i < 10; i++)
    {
        //synchronous operations
    }
}

Will actually marking M2 as async despite it has only sync operations inside will make any difference regarding a way it’s being executed?

>Solution :

No, moreover everything before first await on "truly async" function will be executed syncroniously. Check out the next code:

var task = M1();
Console.WriteLine("before outer await");
await task;
Console.WriteLine("after outer await");

async Task M1()
{
    for (int i = 0; i < 2; i++)
    {
        Console.WriteLine("before await M2");
        await M2();
        Console.WriteLine("after await M2");
    }

    Console.WriteLine("before await");
    await Task.Delay(100);
    Console.WriteLine("after await");
}

async Task M2()
{
    for(int i = 0; i < 10; i++)
    {
        Console.WriteLine(i);
    }
}

Which will produce output like:

before await M2
// ... numbers
after await M2 
before await M2
// ... numbers
after await M2
before await
before outer await
after await
after outer await

And then compare output with the following M2 version:

async Task M2()
{
    await Task.Delay(100);
    for(int i = 0; i < 10; i++)
    {
        Console.WriteLine(i);
    }
}

Which will result in before outer await written earlier (somewhere after first "before await M2").

Some potentially useful links:

• Asynchronous Programming in .NET – Introduction, Misconceptions, and Problems
• Asynchronous programming doc
• Task asynchronous programming model
• How Async/Await Really Works in C# – if you want to dive deeper
• Dissecting the async methods in C# – if you want to dive deeper