CODEWITHSUNDEEP
wpfc#-4.0dependency-properties
Vignesh.N
Vignesh.N

Reputation: 2666

How to bind property to getter-only CLR Property in WPF?

For some purpose I wanted to raise an event whenever the LineCount of a multiline textbox changes
I thought I could achieve this if I bind a dependency property to TextBox.LineCount
Below is the code sample

XAML

<local:MyTextBox Margin="5,65,5,5" x:Name="txtBox" 
       AcceptsReturn="True" AcceptsTab="False" 
       MyProperty="{Binding LineCount, RelativeSource={RelativeSource Self}}" />

Code-Behind

public class MyTextBox : TextBox
{
    public MyTextBox()
    {
        DataContext = this;
    }

    public int MyProperty
    {
        get
        {
            return (int)this.GetValue(LineCountProperty);
        }
        set
        {
            this.SetValue(LineCountProperty, value);
        }
    }

    public static readonly DependencyProperty LineCountProperty =
        DependencyProperty.Register(
        "MyProperty",
        typeof(int),
        typeof(TextBox),
        new FrameworkPropertyMetadata(
            new PropertyChangedCallback(
             (dp, args) => { MessageBox.Show(args.NewValue.ToString()); })
        ));
}

The messagebox is shown only when the form is loaded and not after the line count changes.
However if I change the Binding for the MyProperty from TextBox's LineCount to Text The PropertyChangedEvent is fired everytime the text changes.
I am sure I can fire some custom LineCount changed event from TextChanged eventhandler, but I want to do this via DepedencyProperties for I believe it'll be more efficient.

Upvotes: 2

Views: 845

Answers (1)

ColinE
ColinE

Reputation: 70142

The problem is that LineCount is not a dependency property, see the MSDN documentation. For this reason it does not support binding and will not provide change notification.

Considering that you are making a custom control, do you really need to use binding? Why not just handle the TextChanged event internally and detect when the line count changes manually.

Upvotes: 2

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