A 'Binding' can only be set on a DependencyProperty of a DependencyObject

From a custom control based on TextBox, I created a property named Items, in this way:

public class NewTextBox : TextBox
{
    public ItemCollection Items { get; set; }
}

When using the custom control in XAML, I cannot bind the property because it raises exception "A 'Binding' can only be set on a DependencyProperty of a DependencyObject.".

How do I solve this exception?

Solution 1:

As a side note, it is also worth noting that you will get these binding errors if you copy and paste between objects and forget to change the second typeof(Object) statement.

I couldn't figure out for a good hour why I was getting this error as everything appeared to be defined and correct. I'd moved my properties into a usercontrol as I wanted to go from a single set to a list. Thus:

public static readonly DependencyProperty FoldersProperty = DependencyProperty
    .Register("Folders", typeof(OutlookFolders), typeof(MainWindow),
    new FrameworkPropertyMetadata(new OutlookFolders()));

public OutlookFolders Folders
{
    get { return GetValue(FoldersProperty) as OutlookFolders; }
    set { SetValue(FoldersProperty, value); }
}

Should have become:

public static readonly DependencyProperty FoldersProperty = DependencyProperty
    .Register("Folders", typeof(OutlookFolders), typeof(SavedFolderControl), 
    new FrameworkPropertyMetadata(new OutlookFolders()));

public OutlookFolders Folders
{
    get { return GetValue(FoldersProperty) as OutlookFolders; }
    set { SetValue(FoldersProperty, value); }
}

Until I did this change I kept receiving the error: A 'Binding' cannot be set on the property 'Folders' of type 'SavedFolderControl'. A 'Binding' can only be set on a DependencyProperty of a DependencyObject.

Solution 2:

To solve this exception you need to change the property Items and add a DependencyProperty that will work as a "link" in XAML. The class will be:

public class AutocompleteTextBox : TextBox
{
    public ItemCollection Items
    {
        get {
            return (ItemCollection)GetValue(ItemsProperty); }
        set {
            SetValue(ItemsProperty, value); }
    }

    public static readonly DependencyProperty ItemsProperty =
        DependencyProperty.Register(
            "Items",
            typeof(ItemCollection),
            typeof(AutocompleteTextBox),
            new PropertyMetadata(default(ItemCollection), OnItemsPropertyChanged));

    private static void OnItemsPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        // AutocompleteTextBox source = d as AutocompleteTextBox;
        // Do something...
    }

Solution 3:

Here's another gotcha: Ensure that the string in the first argument of DependencyProperty.Register() matches the name of the related property.

public static readonly DependencyProperty ItemsProperty =
    DependencyProperty.Register(
        "TheItems", // This is wrong
        typeof(ItemCollection),
        typeof(AutocompleteTextBox),
        new PropertyMetadata(default(ItemCollection), OnItemsPropertyChanged));

I ran into this issue when I renamed my property without changing the string.