WPF-Prism CanExecute method not being called

Question

I am coding a simple login UserControl with two TextBoxes (Username and Password) and a Login button. I want the Login button to be enabled only when the username and password fields are filled in. I am using Prism and MVVM. The LoginViewModel contains a property called LoginCommand that is bound to the Login button. I have a CanLoginExecute() method in my ViewModel but it fires only when the application comes up and then never again. So the Login button is never enabled. What am I missing?

Here's my xaml:

<TextBox x:Name="username"     Text="{Binding Path=Username, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" /> <TextBox x:Name="password"     Text="{Binding Path=Password, UpdateSourceTrigger=PropertyChanged, ValidatesOnDataErrors=True}" /> <Button Content="Login"     cmnd:Click.Command="{Binding LoginCommand}" /> 

Here's my ViewModel

class LoginViewModel : IDataErrorInfo, INotifyPropertyChanged {     public LoginViewModel()     {         this.LoginCommand =             new DelegateCommand<object>(                 this.LoginExecute, this.CanLoginExecute);     }      private Boolean CanLoginExecute(object dummyObject)     {         return (string.IsNullOrEmpty(Username) ||                 string.IsNullOrEmpty(Password)) ? false : true;     }      private void LoginExecute(object dummyObject)     {         if (CheckCredentials(Username, Password))         {             ....         }     }      #region IDataErrorInfo Members      public string Error     {         get { throw new NotImplementedException(); }     }      public string this[string columnName]     {         get         {             string result = null;             if (columnName == "Username")             {                 if (string.IsNullOrEmpty(Username))                     result = "Please enter a username";             }             else if (columnName == "Password")             {                 if (string.IsNullOrEmpty(Password))                     result = "Please enter a password";             }             return result;         }     }      #endregion // IDataErrorInfo Members      #region INotifyPropertyChanged Members      public event PropertyChangedEventHandler PropertyChanged;      void OnPropertyChanged(string propertyName)     {         if (PropertyChanged != null)             PropertyChanged(this, new PropertyChangedEventArgs(propertyName));     }      #endregion // INotifyPropertyChanged Members      #region Properties      private String _username;     public String Username     {         get { return _username; }         set         {             if (value == _username)                 return;             _username = value;             this.OnPropertyChanged("Username");         }     }      private String _password;     public String Password     {         get { return _password; }         set         {             if (value == _password)                 return;             _password = value;             this.OnPropertyChanged("Password");         }     }      public ICommand LoginCommand { get; private set; }      #endregion // Properties } 

Answer 1

It is most likely that the bound control is never asking for the CanExecute state again. You need to call the RaiseCanExecuteChanged method on the DelegateCommand whenever you detect a condition that changes the command's CanExecute state. This signals the bound control to update the CanExecute state.