private async void StartButton_Click(object sender, RoutedEventArgs e)
{
    // ExampleMethodAsync returns a Task<int>, which means that the method
    // eventually produces an int result. However, ExampleMethodAsync returns
    // the Task<int> value as soon as it reaches an await.
    ResultsTextBox.Text += "\n";
    try
    {
        int length = await ExampleMethodAsync();
        // Note that you could put "await ExampleMethodAsync()" in the next line where
        // "length" is, but due to when '+=' fetches the value of ResultsTextBox, you
        // would not see the global side effect of ExampleMethodAsync setting the text.
        ResultsTextBox.Text += String.Format("Length: {0}\n", length);
    }
    catch (Exception)
    {
        // Process the exception if one occurs.
    }
}

public async Task<int> ExampleMethodAsync()
{
    var httpClient = new HttpClient();
    int exampleInt = (await httpClient.GetStringAsync("http://msdn.microsoft.com")).Length;
    ResultsTextBox.Text += "Preparing to finish ExampleMethodAsync.\n";
    // After the following return statement, any method that's awaiting
    // ExampleMethodAsync (in this case, StartButton_Click) can get the 
    // integer result.
    return exampleInt;
}
// Output:
// Preparing to finish ExampleMethodAsync.
// Length: 53292

        private async void button_Click(object sender, RoutedEventArgs e)
        {
            TB_Result.Text = await exampleAsync();
        }

        private async Task<string> exampleAsync()
        {
             return await Task.Run(() =>
             {
                 WebClient w = new WebClient();
                 w.DownloadFile("http://provissy.com", "webPage1");
                 w.DownloadFile("http://microsoft.com", "webPage2");
                 string a = w.DownloadString("http://microsoft.com");
                 return a;
             });
        }

        private async Task exampleAsync()
        {
             // 用 await exampleAsync(); 来调用即可，会立即返回。
             await Task.Run(() =>
             {
                 WebClient w = new WebClient();
                 w.DownloadFile("http://provissy.com", "webPage1");
                 w.DownloadFile("http://microsoft.com", "webPage2");
             });
        }