Question

I have an issue with something that should be very simple databinding scenario. I want to bind a list of items. I want to create a user control put it in a ItemsControl’s template and bind the ItemsControl to some data. I am perfectly happy with one time databinding so I was kind of hoping to avoid learning about dependency properties and all the databinding stuff for this simple scenario.

Here is the XAML for the user control:

<TextBlock>Just Something</TextBlock>

And the code behind:

namespace TestWindowsPhoneApplication
{
    public partial class TestControl : UserControl
    {
        public TestData SomeProperty { get; set; }
        public String SomeStringProperty { get; set; }

        public TestControl()
        {
            InitializeComponent();
        }
    }
}

MainPage.xaml:

<ItemsControl Name="itemsList" ItemsSource="{Binding}">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <t:TestControl SomeStringProperty="{Binding Path=SomeString}"></t:TestControl>
            <!--<TextBlock Text="{Binding Path=SomeString}"></TextBlock>-->
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

Here is MainPage.xaml.cs:

namespace TestWindowsPhoneApplication
{
    public class TestData
    {
        public string SomeString { get; set; }
    }

    public partial class MainPage : PhoneApplicationPage
    {
        // Constructor
        public MainPage()
        {
            InitializeComponent();
            itemsList.DataContext = new TestData[] { new TestData { SomeString = "Test1" }, new TestData { SomeString = "Test2" } };
        }
    }
}

When I run the project I get an error “the parameter is incorrect”. I also tried binding directly to the item with SomeProperty={Binding} since that is what I actually want to do but this causes the same error. If I try doing the same thing with the TextBlock control (the commented line) everything works fine.

How can I implement this simple scenario?

Answer 1

To make a property on your custom control “bindable” you have to make it a dependency property. Check out my answer here for a nice simple example of doing just this on a custom control: passing a gridview selected item value to a different ViewModel of different Usercontrol

public string SomeString
{
    get { return (string)GetValue(SomeStringProperty); }
    set { SetValue(SomeStringProperty, value); }
}
public static readonly DependencyProperty SomeStringProperty =
    DependencyProperty.Register("SomeString", typeof(string), typeof(TestControl),
    new PropertyMetadata(string.Empty, new PropertyChangedCallback(OnSomeStringChanged)));

private static void OnSomeStringChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    ((TestControl)d).OnSomeStringChanged(e);
}

protected virtual void OnSomeStringChanged(DependencyPropertyChangedEventArgs e)
{
    //here you can do whatever you'd like with the updated value of SomeString
    string updatedSomeStringValue = e.NewValue;
}