I am new to asynchronous programming, so after going through some async sample codes, I thought of writing a simple async code
I created a simple Winform application and inside the Form I wrote the following code. But its just not working
private Task<string> methodAsync() {
Thread.Sleep(10000);
return "Hello"; //Error: Cannot implicitly convert type 'string' to 'System.Threading.Tasks.Task<string>'
}
private async void button1_Click(object sender, EventArgs e)
{
string s = await methodAsync();
MessageBox.Show(s);
}
Could someone please put some light here..
Best Answer
The listed return type of the method is
Task<string>
. You're trying to return astring
. They are not the same, nor is there an implicit conversion from string toTask<string>
, hence the error.You're likely confusing this with an
async
method in which the return value is automatically wrapped in aTask
by the compiler. Currently that method is not an async method. You almost certainly meant to do this:There are two key changes. First, the method is marked as
async
, which means the return type is wrapped in aTask
, making the method compile. Next, we don't want to do a blocking wait. As a general rule, when using theawait
model always avoid blocking waits when you can.Task.Delay
is a task that will be completed after the specified number of milliseconds. Byawait
-ing that task we are effectively performing a non-blocking wait for that time (in actuality the remainder of the method is a continuation of that task).If you prefer a 4.0 way of doing it, without using
await
, you can do this:The first version will compile down to something that is more or less like this, but it will have some extra boilerplate code in their for supporting error handling and other functionality of
await
we aren't leveraging here.If your
Thread.Sleep(10000)
is really meant to just be a placeholder for some long running method, as opposed to just a way of waiting for a while, then you'll need to ensure that the work is done in another thread, instead of the current context. The easiest way of doing that is throughTask.Run
:Or more likely: