title: Start may not be called on a promise-style task. exception is coming
body: <p>I am creating a simple wpf desktop application. UI have just a button and code in .cs file like.</p>
  
  <pre><code>private void Button_Click_2(object sender, RoutedEventArgs e)
  {
      FunctionA();
  }
  
  public void FunctionA()
  {
      Task.Delay(5000).Start();
      MessageBox.Show("Waiting Complete");
  }
  </code></pre>
  
  <p>But surprisingly line <code>Task.Delay(5000).Start();</code> is throwing an <code>InvalidOperationException</code>: </p>
  
  <blockquote>
    <p>Start may not be called on a promise-style task. </p>
  </blockquote>
  
  <p>Can any one help why it is like this?</p>