Pass Data from Child Window to MainWindow TextBlock

Question

In this example, MainWindow has a button that opens Window2.

Window2 has a button that writes \"Hello, World!\" to MainWindo

Answer 1

You could declare a delegate in Window 2 and pass a function that can be executed when the button is pressed in window 2.

 public delegate void SendMessage(string Message);
 public SendMessage mainWindowDel = null;

Make your constructor that accepts the delegate

 public Window2(SendMessage del)
        {
            mainWindowDel = del;
        }

create window 2 by passing a function as the parameter

 public void showMessage(string Message)
 {
                if(!string.IsNullOrWhiteSpace(Message))
                textBlockMessage.Text = Message;
 }
 private void buttonWindow2_Click(object sender, RoutedEventArgs e)
 {
            // Open Window 2
            //Window2 window2 = new Window2(this);
            Window2 window2 = new Window2(showMessage);
            window2.Left = Math.Max(this.Left - window2.Width, 0);
            window2.Top = Math.Max(this.Top - 0, 0);
            window2.ShowDialog();
  }

call the delegate when they press the button

// Write Message to MainWindow
        private void buttonMessage_Click(object sender, RoutedEventArgs e)
        {
           mainWindowDel("Hello, world!");
        }

Answer 2

You could probably set the datacontext of the MainWindow to an object that you could also pass to Window2 when it's created and set it's datacontext to the same object.

In that object could create a string property that could be used in both windows. And if you implement the INotifyPropertyChanged interface both windows would know when that string is updated.

Answer 3

The answer you're looking for is very implementation-based and depends heavily on what you want Window2 as a class to do.

---

private MainWindow mainwindow;

This is acceptable.

public MainWindow mainwindow { get; private set; }

This would work but doesn't respect naming conventions because it's a property. Usually you'd use this for encapsulation of a field or for easy access to a computed value.

public Window MainWindow { get; set; }

This is not acceptable in your context because Window does not contain a textBlockMessage.

private object mainwindow { get; private set; };

This also wouldn't work for the same reason as above.

private MainWindow mainwindow = ((MainWindow)System.Windows.Application.Current.MainWindow);

This would work and would even let you not keep a field for the reference to the MainWindow instance in your Window2 instances. Still needs to get that MainWindow everytime you click the button however.

---

Another interesting way to do what you're doing however is to simply pass the handler to the child windows at instanciation:

MainWindow

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    // Open Window 2
    private void buttonWindow2_Click(object sender, RoutedEventArgs e)
    {
        Window2 window2 = new Window2(); // No need to give a reference to the child window anymore
        window2.setClickHandler((obj, ev) => {
            textBlockMessage.Text = "Hello, world!"; // Direct access to the textblock.
        });
        window2.Left = Math.Max(this.Left - window2.Width, 0);
        window2.Top = Math.Max(this.Top - 0, 0);
        window2.ShowDialog();
    }
}

Window2

public partial class Window2 : Window
{
    public Window2()
    {
        InitializeComponent();
    }

    public void setClickHandler(RoutedEventHandler handler)
    {
        // The handler is given to the click event.
        buttonMessage.Click -= handler;
        buttonMessage.Click += handler;
    }
}

And with that your Window2 class has no need to know MainWindow.

Answer 4

Another Approach

MainWindow

 public partial class MainWindow : Window
    {

        public MainWindow()
        {
            InitializeComponent();
        }

        private bool DisplayText(string displayText)
        {
            txt_Main.Text = displayText;
            return true;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            Window2 win2 = new Window2(DisplayText);
            win2.ShowDialog();
        }
    }

Window2

 public partial class Window2 : Window
    {
        private Func<string, bool> mainWindowMethod;

        public Window2(Func<string, bool> displayMethod)
        {
            InitializeComponent();
            this.mainWindowMethod = displayMethod;
        }

        private void Button_Click(object sender, RoutedEventArgs e)
        {
            this.mainWindowMethod("Hello World");
        }
    }

Answer 5

The 'proper' way usually depends on what your needs and circumstances are. But in general, using a delegate to pass data between windows is a common and standard practice.

Lets say the data you want to pass is a string. In your Main window, you want to create a delegate that lets you pass a string. Then you create an instance of that delegate type and subscribe a method that matches. Then when you open your secondary window, you pass that delegate to your secondary window.

public delegate void DataTransfer(string data);

public partial class MainWindow : Window
{
    public DataTransfer transferDelegate;

    public MainWindow()
    {
        InitializeComponent();
        transferDelegate += new DataTransfer(DataMethod);
    }

    public void DataMethod(string data)
    {
        // Do what you want with your data.
    }

    private void button1_Click(object sender, EventArgs e)
    {
        Window2 win = new Window2(transferDelegate);
        win.Show();
    }
}

Now, when you invoke that delegate in your secondary window, the DataMethod() of your Form1 gets called, and so you can pass information between windows.

Your secondary window implementation should look like this:

public partial class Window2 : Window
{
    DataTransfer transferDel;

    public Window2(DataTransfer del)
    {
        InitializeComponent();
        transferDel = del;
    }

    private void button1_Click(object sender, EventArgs e)
    {
        string data = "Hello, World!"; // Your string data to pass.
        transferDel.Invoke(data);
    }
}

As you can see, when you invoke the delegate that was passed, it calls the corresponding method in your main program.

One stand out advantage of this method is that you don't need to pass an instance of MainWindow to your Window2, you simply use delegates and subscribed methods to pass data between the two instances of windows.