Why is the Dependency Property not returning its value?

Question

I have a MyUserControl with the following Xaml:

<TextBox Text="{Binding InputValueProperty}" />

In the MyUserControl.xaml.cs I have:

public string InputValue
{
    get { return (string)GetValue(InputValueProperty); }
    set { SetValue(InputValueProperty, value); }
}
public static readonly DependencyProperty InputValueProperty = 
    DependencyProperty.Register("InputValueProperty", typeof(string), 
    typeof(MyUserControl));

In my MainWindow.xaml I create a user control:

<local:MyUserControl InputValue="My Input" />

Later on in my MainWindow.xaml.cs I am trying to access this string. All instances of MyUserControl are contained in a List and I access them with a foreach.

string temp = userControl.InputValue;

This is always null. In my MainWindow.xaml I can see the "My Input" in the text box of the user control but I can't ever seem to get it out of there.

Answer 1

DependencyProperty.Register("InputValueProperty", ...

That should be:

DependencyProperty.Register("InputValue", ...

XAML depends on the registered name of the property, not the name of the property accessor.

Answer 2

It looks like the problem is in your binding. Here's a working example that's modeled off your code with a relative source binding:

Here's the user control:

  public partial class MyUserControl : UserControl
  {
    public MyUserControl()
    {
      InitializeComponent();
    }

    public string InputValue
    {
      get { return (string)GetValue(InputValueProperty); }
      set { SetValue(InputValueProperty, value); }
    }
    public static readonly DependencyProperty InputValueProperty =
        DependencyProperty.Register("InputValueProperty", typeof(string),
        typeof(MyUserControl));
  }
<UserControl x:Class="WpfApplication4.MyUserControl"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:WpfApplication4" Height="30" Width="300">
    <Grid>
       <TextBox Text="{Binding Path=InputValue, RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type local:MyUserControl}}}" />
    </Grid>
</UserControl>

And here's the window:

  public partial class Window1 : Window
  {
    public Window1()
    {
      InitializeComponent();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
      string text1 = ctrl1.InputValue;
      string text2 = ctrl2.InputValue;
      string text3 = ctrl3.InputValue;
//breakpoint here
    }
  }

<Window x:Class="WpfApplication4.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" xmlns:local="clr-namespace:WpfApplication4" Title="Window1" Height="300" Width="300">
    <Grid>
    <StackPanel>
      <local:MyUserControl x:Name="ctrl1" InputValue="My Input" />
      <local:MyUserControl x:Name="ctrl2" InputValue="2" />
      <local:MyUserControl x:Name="ctrl3" InputValue="3" />
      <Button Click="Button_Click" Height="25" Content="debug"/>
      </StackPanel>
    </Grid>
</Window>

If i throw a breakpoint in the click event i can see the bound values of each of the controls. (if you copy and paste from this be sure to change WpfApplication4 to whatever your project is called.

Answer 3

You need to implement INotifyPropertyChanged on your class that has the property

    public class YourClassThatHasTheInputValuePropertyInIt: INotifyPropertyChanged
    {
            public event PropertyChangedEventHandler PropertyChanged;
            private void NotifyPropertyChanged(String propertyName)
            {
                if (PropertyChanged != null)
                    PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }

            public string InputValue
            {
                get { return (string)GetValue(InputValueProperty); }
                set { SetValue(InputValueProperty, value);
                      NotifyPropertyChanged("InputValue"); }
     }
    }

This will allow the binding to pick up the property