InvalidOperationException when do a process twice

I had to write an async method to contact with web service. This is my method in WebServiceHelper class: ``` public static Task<int> SignIn(string username, string password) { try { TaskCompletionSource<int> tcs = new TaskCompletionSource<int>(); service.LoginCompleted += (object sender, WebService.LoginCompletedEventArgs e) => { if (e.Error != null) tcs.SetResult(-1); else tcs.SetResult((int)e.Result); }; service.LoginAsync(username, password); return tcs.Task; } catch (Exception ex) { MessageBox.Show("Error: " + ex.Message); return null; } } ``` Then I call it in a button clicked event like this: ``` private async void btLogIn_Click(object sender, RoutedEventArgs e) { try { int si = await WebServiceHelper .SignIn(tbUsername.Text, tbPassword.Text); if (si != 0) MessageBox.Show("Signed in successfully!"); else MessageBox.Show("Couldn't sign in"); } catch (Exception ex) { MessageBox.Show(ex.Message); } } ``` It worked fine on the first time I clicked the button, but when I did in again, an error came up: "InvalidOperationException, An attempt was made to transition a task to a final state when it had already completed." I did some little search and found something here: Tasks seem to start automatically I understand that I should do something to stop the process before start it again but I don't know exactly how and why. Can someone please explain this for me?