public AuthViewModel() { SignInCommand = new DelegateCommand <AuthDialog>(authDialog => { if (!string.IsNullOrEmpty(Login) && !string.IsNullOrEmpty(authDialog.PasswordBox.Password)) { _model.Login = Login; _model.Password = authDialog.PasswordBox.Password; if (_model.SignIn()) { // Close current window and open MainWindow. var mainWindow = new MainWindow(); mainWindow.Show(); authDialog.Close(); } } else { MessageBox.Show("Please fill all required fields!"); } }); SignUpCommand = new DelegateCommand <AuthDialog>(authDialog => { if (!string.IsNullOrEmpty(Name) && !string.IsNullOrEmpty(Login) && !string.IsNullOrEmpty(authDialog.PasswordBox.Password)) { _model.Name = Name; _model.Login = Login; _model.Password = authDialog.PasswordBox.Password; if (_model.SignUp()) { // Close current window and open MainWindow. var mainWindow = new MainWindow(); mainWindow.Show(); authDialog.Close(); } else { MessageBox.Show("Invalid credentials."); // Or no internet connection. } } else { MessageBox.Show("Please fill all required fields!"); } }); }