Deadlock with async/await

问题

Suppose I'm writing a custom MVC filter which does some asynchronous calls within the method overrides, like so:

public class MyActionFilter : System.Web.Mvc.ActionFilterAttribute
{
  public override void OnActionExecuted(ActionExecutedContext ctx)
  {
    var stuff = ConfigureAwaitHelper1().Result;
    // do stuff
  }

  public override void OnActionExecuting(ActionExecutingContext ctx)
  {
    var stuff = ConfigureAwaitHelper2().Result;
    // do stuff
  }

  private async Task<string> ConfigureAwaitHelper1()
  {
    var result = await client.GetAsStringAsync("blah.com").ConfigureAwait(false);
    return result;
  }

  private async Task<string> ConfigureAwaitHelper2()
  {
    return await client.GetAsStringAsync("blah.com").ConfigureAwait(false);
  }
}

Why does OnActionExecuting deadlock, whereas OnActionExecuted does not? I don't see the fundamental difference between the two. The act of returning happens only after the asynchronous task is complete, which is rather like putting the result into an "anonymous return" local var before returning it, so I don't see why the former should deadlock.

回答1:

Why does OnActionExecuting deadlock, whereas OnActionExecuted does not?

I'm surprised it works at all. The reason you're experiencing the deadlock is due to the fact that you're invoking a .Result on a Task. This is evil and you should only ever invoke .Result and .Wait in console applications.

来源：https://stackoverflow.com/questions/38446331/deadlock-with-async-await