A Change event for Dependency Properties

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WPF comes with Dependency Properties and everybody using WPF will know about these new kind of properties. When you define your own Dependency Properties in your own class you can pretty easy add a property change event handler:

public int MyProperty
    get { return (int)GetValue(MyPropertyProperty); }
    set { SetValue(MyPropertyProperty, value); }
public static readonly DependencyProperty MyPropertyProperty =
                        new UIPropertyMetadata(0,

private static void MyPropertyvalueChangeCallback
                        (DependencyObject d,
                         DependencyPropertyChangedEventArgs e)

But how to add such a event handler to an already existing Dependency Property or somewhere else then in the defining class? E.g. to an property of a WPF-Control that was not build by you. Take a standard WPF-TextBox; both the Text and the FontSize properties are Dependency Properties but the TextBox-class only provides a change event for the Text-property. Nevertheless you can get a change event for any Dependency Property:

DependencyPropertyDescriptor dpd =
        (Control.FontSizeProperty, typeof (TextBox));
dpd.AddValueChanged(someTextBox, SomeTextBoxFontSizeChanged);

Every time the FontSizeProperty on the instance someTextBox changes the given method is called. It’s that easy and you can implement this code everywhere not only within the class that defines the property.

Foxit Reader vs. Acrobat Reader: the default program problem

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I’m a big fan of the Foxit pdf Reader. It is just so damn fast in comparison to Acrobat Reader when opening pdf documents. Yesterday I had to go through a bunch of pdfs each containing only one or two pages to find a specific one. While waiting for Acrobat Reader to open a file I could open, check an close a few files with Foxit. If you are not using Foxit Reader give it a try…

But for some pdf features you will need the original Adobe Reader. E.g. some documents with fields to enter data, with scripts or DRM secured files won’t open in Foxit.

I always have both installed on my machine and Foxit is the default application for opening pdf documents. But this setting changed some time ago on my Windows 7 64 bit machine.

I reset the default program for pdf in windows control panel to Foxit and all icons of pdf documents changed to the Foxit one. I can double click a file and Foxit opens  – great. But in the very moment Foxit comes up the default program is set to Acrobat Reader again. This happens all the time.

I’m not sure what’s going on here and what is involved. I was blaming Acrobat and searched for a solution on the Acrobat side of the problem. Nothing.

In the end an update of Foxit to the current version fixes my problems.

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