在C＃中的async-await块中使用“ out”时，不清楚编译器错误

Question

I have a method I'm calling and I want to make sure it absolutely finishes before continuing. I'm also interested in knowing if it completed successfully or not, so I'm doing the following:

async void Foo() {
  bool success;

  // stuff

  await Task.Run(
    () => Bar(out success)
  );

  if (!success) { // this is the line causing the compiler-error
    // handle
  }

  // other stuff
}

void Bar(out bool success);

But I'm getting error

CS0165 Use of unassigned local variable 'success'

This isn't a huge deal, as I can get cute and initialize success=false and pass it as ref instead of out and that seems to work as desired. However, I'm curious about what the intricacies of async-await (or Task.Run ) are that seem to lead to a case that doesn't guarantee out success will be properly assigned.

Any enlightenment is appreciated!

Edit:

To add a bit more context, the following block compiles and executes fine.

void Caller() {
  bool success;
  Callee(out success);
  if (success) {
    // do something
  }
}

void Callee();

This is because out parameters are not required to be initialized before being passed. For more on out : https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/out-parameter-modifier

Answer 1

Consider the following possible (though totally incorrect) implementation of Task.Run :

public class Task
{
    public static Task Run(Action action)
    {
        return Task.Delay(500);
    }
    //...
}

Now think about the code in question. With this implementation of Task.Run , will the Bar method ever get called? No. Therefore success will never get initialized, and an attempt to access an uninitialized variable is made.

The compiler has no idea about the real implementation of Task.Run or what it will do. It can't assume that it isn't implemented like I have above, and therefore an uninitialized variable is possible.