等待/异步参考错​​误

Question

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?

Answer 1

• 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)

Answer 2

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.

Answer 3

If zzztest is a long running operation, you can do this to run it in a background thread:

async private Task<string> thr_calc(object sender, EventArgs e)
{
   return await Task.Run<string>(() => zzztest());            
}

The above should solve your compile errors as well.

If zzztest is NOT a long running operation, then consider NOT using await / async .