Can not bind textblock property from another class to UI class using MVVM

I'm a beginner of sliverlight and MVVM. I can not bind textblock property from another class to UI class using MVVM.

My code is here. Please let me know how to bind textblock property in bellow Authentication.cs.

MainPage.xaml

<TextBlock Height="30" Margin="122,218,0,0" Name="textBlock3" Text="{Binding Path=ErrorStatus, Mode=TwoWay}" VerticalAlignment="Top" HorizontalAlignment="Left" Width="86" />

MainPage.xaml.cs

private Authentication authentication;

// Constructor
public MainPage()
{
    InitializeComponent();
    this.DataContext = authentication;
}

ViewModelBase.cs

public class ViewModelBase : INotifyPropertyChanged
{
    public 开发者_如何学运维event PropertyChangedEventHandler PropertyChanged;

    protected void NotifyPropertyChanged(String info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }
}

Authentication.cs

public class Authentication : ViewModelBase
{
    private string _ErrorStatus;
    public string ErrorStatus
    {
        get
        {
            return _ErrorStatus;
        }
        set
        {
            _ErrorStatus = value;
            NotifyPropertyChanged("ErrorStatus");
        }
    }

    void Authenticate()
    {
        //Here, I write authentication code....
        //After the authentication code, I want to change textBlock property depend on auth status.
        //Please let me know how to write code.
    }
}

You write ErrorStatus = "Access Denied"; in your Authenitcate() method

The TextBox's Text is pointing to the ErrorStatus property, so anytime it updates the TextBox will also get updated automatically.

The only thing you need to be sure of is that you call Authenticate() on the same object that the TextBox is bound to.

private Authentication authentication = new Authentication();

public MainPage()
{
    InitializeComponent();

    // New line here
    this.authentication = new Authentication();

    this.DataContext = authentication;
}

void btnAuthenticate_Click(object src, EventArgs e)
{
    authentication.Authenticate();
}

XAML:

<TextBlock Text="{Binding Path=ErrorStatus, Mode=TwoWay}" />

You haven't created a new instance of Authentication. Add the following line to the main window constuctor:

// Constructor
public MainPage()
{
    InitializeComponent();

    // New line here
    this.authentication = new Authentication();

    this.DataContext = authentication;
}

When you call Authenticate(), you can just assign a new value to ErrorStatus and it should show up in the TextBlock.

This is where the delegate command pattern (also called relay command) comes into play http://kharasoft.wordpress.com/2007/10/16/the-delegating-command/ using this pattern, instead of handling button click in your code behind, your viewmodel exposes an instance of ICommand that does little more than routing the events to a method on the viewmodel. Now authenticate runs in the context of your viewmodel and can update the property directly.