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im trying to do some async operation in some function returning string.
async private void button1_Click(object sender, EventArgs e)
{
string output = await thr_calc(this, null);
}
async private Task<string> thr_calc(object sender, EventArgs e)
{
return await zzztest();
}
string zzztest()
{
string asd;
//some stuff here
return asd;
}
But it gives me errors on each string contains words async/await! Im using russian version of ms vs express 2012 for windows desktop, so here is the translation of errors:
Cannot find all types required by the 'async' modifier. Are you targeting the wrong framework version, or missing a reference to an assembly?
And 2 errors:
Predefined type 'System.Runtime.CompilerServices.IAsyncStateMachine' is not defined or imported
I cant find that reference. I've tried to use async/await before and it worked fine, now im doing all the same and its not. What I am missing?
804
The last way to handle an error with async/await is called a higher order function. We have talked about this a couple of times now. A higher order function is a function that returns another function. The way it works is you go ahead and define all of your functions, just as if you were never to have any errors.
If a promise resolves normally, then await promise returns the result. But in the case of a rejection, it throws the error, just as if there were a throw statement at that line. In real situations, the promise may take some time before it rejects. In that case there will be a delay before await throws an error.
To recap: Throwing error from an async function won't spit out a "plain exception". Async functions and async methods always return a Promise, either resolved or rejected. To intercept exceptions from async functions you must use catch() .
On a .NET 4.0 project, I resolved 'Predefined type 'System.Runtime.CompilerServices.IAsyncStateMachine' is not defined or imported' by installing the Microsoft.Bcl.Async package from nuget. In VS's nuget package manager, search for 'bcl' and install the async-looking one.
114
In thr_calc, use:
return zzztest()
Also, make sure you've set your project to use .Net 4.5 or later (that's when "async" was introduced)
37
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