使用.net 4中的任务等待事件触发

Question

I have function A that calls function B from an external class that I cannot modify. Function B fires an event when completed on which I subscribed the doSomeStuff function. I only want to finish Function A when doSomeStuff is finished. I can only use .Net 4, so no use of async etc.. I currently have this setup which freezes my program;

static EventWaitHandle _WaitHandle = new AutoResetEvent(false);

public void functionA()
{
  functionB.completed += doSomeStuff;
  Task runFunctionB = Task.Factory.StartNew(() => { functionB(); });
  _WaitHandle.WaitOne(); 
}

public void doSomeStuff(object sender, EventArgs e)
{
  // do stuff
  WaitHandle.Set();
}

I would expect task runFunctionB to execute as a seperate thread and therefore ending at my doSomeStuff function, but it seems not to do anything...?

Answer 1

Instead of WaitHandle you can use TaskCompletionSource<T> and wait for TaskCompletionSource<T>.Task .

TaskCompletionSource<object> completionSource = new TaskCompletionSource<object>();

public void functionA()
{
  functionB.completed += doSomeStuff;
  Task runFunctionB = Task.Factory.StartNew(() => { functionB(); });

  completionSource.Task.ContinueWith((result)=> 
  {
     //Do whatever
  });
}

public void doSomeStuff(object sender, EventArgs e)
{
    // do stuff
    completionSource.SetResult(null);
}

I can only use .Net 4, so no use of async etc..

That is a misconception. You can use async-await in .Net 4.0 using BCL.Async nuget package given that you have Visual studio 2012 or higher still targeting .Net 4.0.

In which case you can await the Task.

public async Task functionA()
{
  functionB.completed += doSomeStuff;
  Task runFunctionB = Task.Factory.StartNew(() => { functionB(); });

  await completionSource.Task;
  //Do whatever; event is raised
}