Dependency Property in F# Default Value does not match

Question

I am trying to convert a C# Dependency Property that limits the maximum length of text entered into a ComboBox to F#. The program is a MVVM program that uses F# for the model and viewmodel, and C# for the view. the working C# code is this:

public class myComboBoxProperties
    {
        public static int GetMaxLength(DependencyObject obj)
        {
            return (int)obj.GetValue(MaxLengthProperty);
        }

        public static void SetMaxLength(DependencyObject obj, int value)
        {
            obj.SetValue(MaxLengthProperty, value);
        }

        // Using a DependencyProperty as the backing store for MaxLength. This enables animation, styling, binding, etc...
        public static readonly DependencyProperty MaxLengthProperty =
            DependencyProperty.RegisterAttached("MaxLength",
            typeof(int),
            typeof(myComboBoxProperties),
            new UIPropertyMetadata(OnMaxLengthChanged));

        private static void OnMaxLengthChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
        {
            if (obj is ComboBox)
            {
                ComboBox comboBox = (ComboBox)obj;

                comboBox.Loaded += (sender, e) =>
                {
                    TextBox textBox = comboBox.Template.FindName("PART_EditableTextBox", comboBox) as TextBox;

                    if (textBox != null)
                    {
                        textBox.SetValue(TextBox.MaxLengthProperty, args.NewValue);
                    }
                };
            }
        }
    }

The F# code is this:

type myComboBoxProperties() =

    static let OnMaxLengthChanged  (myobj1 : DependencyObject, args : DependencyPropertyChangedEventArgs)  =

        let comboBox = myobj1 :?> ComboBox

        comboBox.Loaded.Subscribe (fun _ -> 
                                    let textBox : TextBox = comboBox.Template.FindName("PART_EditableTextBox", comboBox) :?> TextBox 
                                    match textBox with
                                    | null -> ()
                                    |_ -> textBox.SetValue(TextBox.MaxLengthProperty, args.NewValue))

    static let MaxLengthProperty = DependencyProperty.RegisterAttached("MaxLength", typeof<int>, typeof<myComboBoxProperties>, new UIPropertyMetadata(OnMaxLengthChanged))

    static member GetMaxLength (myobj : DependencyObject) = myobj.GetValue(MaxLengthProperty) :?> int

    static member SetMaxLength (myobj : DependencyObject, value : int) = myobj.SetValue(MaxLengthProperty, value)

The problem I am having is that the XAML error I get is:

Default value type does not match type of property MaxLength

What am I doing wrong?

Answer 1

You could try this

open System.Windows
open System.Windows.Controls

type MyComboBoxProperties() =

  static let OnMaxLengthChanged  (myobj1 : DependencyObject) (args : DependencyPropertyChangedEventArgs) =

    let comboBox = myobj1 :?> ComboBox

    comboBox.Loaded.Add (
      fun _ -> 
        let textBox : TextBox = comboBox.Template.FindName("PART_EditableTextBox", comboBox) :?> TextBox 
        match textBox with
        | null -> ()
        |_ -> textBox.SetValue(TextBox.MaxLengthProperty, args.NewValue)
      )

  static let MaxLengthProperty = 
    DependencyProperty.RegisterAttached(
      "MaxLength", 
      typeof<int>, 
      typeof<MyComboBoxProperties>, 
      UIPropertyMetadata(0, PropertyChangedCallback OnMaxLengthChanged)
      )

  static member GetMaxLength (myobj : DependencyObject) = myobj.GetValue(MaxLengthProperty) :?> int

  static member SetMaxLength (myobj : DependencyObject, value : int) = myobj.SetValue(MaxLengthProperty, value)

The key difference to your code is this UIPropertyMetadata(0, PropertyChangedCallback OnMaxLengthChanged) that converts OnMaxLengthChanged into a PropertyChangedCallback.

But it feels odd to me that you subscribe to the .Loaded even each time the max value you change. I suspect you only like to subscribe the first time?