C＃WPF UserControl设置属性

Question

Hi i have made my own UserControl, its a little windows explorer.

i defined a Property in the Control that sets the Path where the Explorer should start from listing the Directory:

 public string SetRootPath
    {
        get { return rootPath; }
        set { rootPath = value; }
    }

and im binding the TreeView that i have with a method "listDirectory"

 public UserControl1()
        {
            InitializeComponent();
            this.DokumentBrowser.ItemsSource = listDirectory(SetRootPath);
        }

when im calling it and i try to set the SetRootPath Property to a local path

<mycontrol:UserControl1 SetRootPath="c:\\temp" />

the Variabel SetRootPath is everytime null and i get an Exception because nothing is assigned. So why is the Property never setted with the value that i assign?

regards

Answer 1

The XAML parser first constructs the user control and then sets the SetRootPath property. Therefore, SetRootPath is null in UserControl1's constructor. You should move the line

 this.DokumentBrowser.ItemsSource = listDirectory(SetRootPath); 

to a later point in the lifecycle of UserControl1. Or use a dependency property instead, and write an OnPropertyChanged handler. (See http://msdn.microsoft.com/en-us/library/ms752914.aspx ).

Answer 2

You are accessing SetRootPath in the constructor. At that point in time, XAML hasn't yet had the chance to set your property, so it's still null . Try to set the ItemsSource of your DocumentBrowser at a later time in the UserControl life cycle. A good choice would be the setter of SetRootPath .

(In fact, there are a few more "WPF-like" options for doing this:

Option A: Make SetRootPath a dependency property and change DocumentBrowser.ItemsSource during its PropertyChanged callback.

Option B: Like Option A, but don't handle PropertyChanged. Instead, bind the DocumentBrowser's ItemsSource property to your SetRootPath property, using a converter which applies listDirectory .)

PS: I'd call it RootPath , not SetRootPath .