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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") {

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