Question

i have a customcontrol

---

    static CustomControl1()
    {
        DefaultStyleKeyProperty.OverrideMetadata(typeof(CustomControl1), new     FrameworkPropertyMetadata(typeof(CustomControl1)));
    }

    public List<string> MyProperty
    {
        get { return (List<string>)GetValue(MyPropertyProperty); }
        set { SetValue(MyPropertyProperty, value); }
    }
    public static readonly DependencyProperty MyPropertyProperty =
        DependencyProperty.Register("MyProperty", typeof(List<string>), 
                                                   typeof(CustomControl1), 
                                                   new UIPropertyMetadata(new List<string>()));        

---

When I use more than one of the CustomControl1 in my application and set value for each MyProperty

  <StackPanel HorizontalAlignment="Left" Orientation="Vertical" VerticalAlignment="Top" Width="176">
        <local:CustomControl1>          
            <local:CustomControl1.MyProperty>
                <System:String>A</System:String>
                <System:String>B</System:String>
                <System:String>C</System:String>
                <System:String>D</System:String>
            </local:CustomControl1.MyProperty>          
        </local:CustomControl1>
        <local:CustomControl1>          
            <local:CustomControl1.MyProperty>
                <System:String>F</System:String>
                <System:String>E</System:String>                    
            </local:CustomControl1.MyProperty>          
        </local:CustomControl1>
    </StackPanel>

when run solution , all values shown in each CustomControl1
and in design mode only show value of last customcontrol1.

So it looks as all of them share the same instance data.

Answer 1

When creating a Dependency Property for a collection (List, Dictionary…) always reinitialize the DP in the class’s constructor.
(Otherwise you’ll be using the same list for all instances)

So in your case:

public CustomControl1()
{
    MyProperty = new List<string>();
}

and remove the value in the Dependency Property’s default value:

public static readonly DependencyProperty MyPropertyProperty =
        DependencyProperty.Register("MyProperty", typeof(List<string>), 
                                                   typeof(CustomControl1), 
                                                   new UIPropertyMetadata(null));