程序不等到按钮单击C＃Windows窗体

Question

这段代码应该等到我单击按钮后，如果条件没有单击任何按钮，它将继续执行。为什么会这样？

TaskCompletionSource<bool> _tcs;

public async Task gCaptcha()
{
    string s = webBrowser1.DocumentText;

    if (s.Contains(captchaPageTxt.Text))
    {
        //MessageBox.Show(captchaPageTxt.Text);
        MessageBox.Show(new Form() { TopMost = true }, "please solve the captcha you have " + captchaSolveWaitTime.Text + " " +"seconds");

          _tcs = new TaskCompletionSource<bool>();
             await _tcs.Task // Here I want to wait until button3 clicked or checkbox1.checked. Then it need to continue with if condition etc.
        if (captchaCompleted.Checked)
        {
            return;
        }
    }
}

void ButtonClicked(object sender, EventArgs e)
{
    _tcs.SetResult(false);
}