Issue regarding asynchronous Programming with Async and Await c#

Question

i was learning how to use Async and Await c#. so i got a link http://msdn.microsoft.com/en-us/library/vstudio/hh191443.aspx#BKMK_WhatHappensUnderstandinganAsyncMethod

from here i try to run the code from VS2012 IDE but getting error. this function is raising error.

private void button1_Click(object sender, EventArgs e)
        {
            int contentLength = await AccessTheWebAsync();

            label1.Text= String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
        }

this line giving error await AccessTheWebAsync(); The 'await' operator can only be used within an async method. Consider marking this method with the 'async' modifier and changing its return type to 'Task'

what i am doing the wrong. please guide me how to run the code. thanks

germi · Accepted Answer · 2013-11-14T08:13:56.307

5

It very clearly states that you have to decorate your method with async. Like so:

// note the 'async'!
private async void button1_Click(object sender, EventArgs e)

Have a look here: async (C# Reference).

By using the async modifier, you specify that a method, lambda expression, or anonymous method is asynchronous. If you use this modifier on a method or expression, it's referred to as an async method.

edited Nov 14 '13 at 08:13

answered Nov 14 '13 at 08:07

germi

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so it means that when we use await keyword inside in any function then function signature must have async keyword...am i right ? – Thomas Nov 14 '13 at 08:46
it was my fault that i did not notice StartButton_Click used async keyword. anyway thanks. – Thomas Nov 14 '13 at 08:49

Pablo Retyk · Answer 2 · 2013-11-14T08:30:30.123

1

You need to put async in your method, I modified your code since the Click event signature does not return int, and your method AccessTheWebAsync does, so I moved it to another method async that returns int, anyway I async and await are kind of syntactic sugar and is recommended that you take a look at what really happens to your code when you use these keywords, take a look here: http://www.codeproject.com/Articles/535635/Async-Await-and-the-Generated-StateMachine

private async void button1_Click(object sender, EventArgs e)
        {
            await ClickAsync();
        }

        private async Task<int> AccessTheWebAsync()
        {

            return await Task.Run(() =>
                {
                    Task.Delay(10000);  //Some heavy work here
                    return 3; //replace with real result
                });

        }

        public async Task ClickAsync()
        {
            int contentLength = await AccessTheWebAsync();

            label1.Text = String.Format("\r\nLength of the downloaded string: {0}.\r\n", contentLength);
        }
    }

edited Nov 14 '13 at 08:30

answered Nov 14 '13 at 08:24

Pablo Retyk

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so it means that when we use await keyword inside in any function then function signature must have async keyword...am i right ? – Thomas Nov 14 '13 at 08:47
yes, you can also call the method without async but this is not recommended because it will run sync, also a warning will be shown for this case – Pablo Retyk Nov 14 '13 at 08:51
can u please tell me what is state machine because your link is about state machine. so if possible then please briefly explain what is state machine. – Thomas Nov 14 '13 at 08:59
@Thomas why don't you post a question on SO like you do for everything/ Or even better... GOOGLE IT! – Alex Nov 14 '13 at 16:50

Issue regarding asynchronous Programming with Async and Await c#

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