error binding isEnabled to button in xaml

Question

I am new to xaml, WPFs, C# and the MVVM paradigm. I have started with an app based on this example project, in the selected excerpts i want to disable the authenticate button from the LoginPageViewModel after the authenticate button has been clicked(There is no point clicking the button if you are authenticated). I have got command binding working, as well as text control binding between the view and ViewModel. my LoginPageViewModel is based on a abstract class that inherits from INotifyPropertyChanged

The setter AuthenticateButtonEnabled is working, but it is not binding to the isEnabled proprerty on the form. My question is, what could I have missed, and How can i trace the binding between a View and a ViewModel?

the LoginPageView.xaml button:

        <Button x:Name="authenticateButton" Content="{x:Static res:Strings.LoginPage_authenticateButton_content}" 
            Grid.Column="2" Margin="53,4,0,10" 
            Grid.Row="2" FontSize="16" 
            IsEnabled="{Binding Path=AuthenticateButtonEnabled}"
            Command="{Binding Path=AuthenticateCommand}" HorizontalAlignment="Left" Width="87"/>

the viewModel

    private String _username;
    private String _responseTextBlock;
    private String _linkTextBlockURI;
    private String _linkTextBlockText;
    private bool _authenticateButtonEnabled;
    ...
    private async void Authenticate()
    {
        ResponseTextBlock = Strings.LoginPage_responseBlock_content_checking;#this works!
        AuthenticateButtonEnabled = false;
        return;

    }
    ....

    public bool AuthenticateButtonEnabled 
    {
        get { return _authenticateButtonEnabled; }
        set { _authenticateButtonEnabled = value;  OnPropertyChanged("AuthenticateButtonEnabled"); }
    }
    // this is in the abstract class.
    protected virtual void OnPropertyChanged(string propertyName)
    {
        PropertyChangedEventHandler handler = this.PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyName));
    }

Answer 1

If you want to have both: command and AuthenticateButtonEnabled, then simply check for this property in CanExecute delegate and vise-versa in property setter update command.

Here is implementation with DelegateCommand and some improvements which you may find useful:

bool _isAuthenticateButtonEnabled;
public bool IsAuthenticateButtonEnabled 
{
    get { return _isAuthenticateButtonEnabled; }
    set
    {
        _isAuthenticateButtonEnabled = value;
        OnPropertyChanged();
        AuthenticateCommand.Update();
    }
}

// the base could class could actually implement this
void OnPropertyChanged([CallerMemberName] string property) => PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(property));

public DelegateCommand AuthenticateCommand { get; }

// view model constructor
public ViewModel()
{
    AuthenticateCommand = new DelegateCommand(o =>
    {
       ... // some actions when command is executed
    }, o =>
    {
       bool somecondition = ...; // some condition to disable button, e.q. when executing command
       return somecondition && IsAuthenticateButtonEnabled;
    });
}

This will let you to have both: property to enable/disable button, which can be used in binding (to another control, e.g. CheckBox.IsChecked) and command which can have independent condition to disable button when command shouldn't be executed (typically in async command delegate, when it performs a long running command, but for this you may want to check this answer.).