How to control the order of Dependecy Properties Initialization in C#

March 17, 2022

I’m trying to create a TextBox control that has a "Table of values" and an "Index" as dependency properties, were I can changed the Index property and the TextBox will look for the corresponding Value from the Table.

Here’s the code:

public class Record
{
    public int Index { get; set; }
    public string Value { get; set; }
}

public class IndexedTextBox : TextBox
{
    public static readonly DependencyProperty TableProperty = DependencyProperty.Register(nameof(Table),
                    typeof(IEnumerable<Record>), typeof(IndexedTextBox));
    public static readonly DependencyProperty IndexProperty = DependencyProperty.Register(nameof(Index),
                    typeof(int), typeof(IndexedTextBox), new PropertyMetadata(0, IndexChangedCallback));

    private static void IndexChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        IndexedTextBox ctl = d as IndexedTextBox;
        int index = (int)e.NewValue;

        if (ctl.Table.Any(t => t.Index == index))
            ctl.Text = ctl.Table.Where(t => t.Index == index).FirstOrDefault().Value;
        else
            ctl.Text = "N/A";
    }

    public IEnumerable<Record> Table
    {
        get => (IEnumerable<Record>)GetValue(TableProperty);
        set => SetValue(TableProperty, value);
    }

    public int Index {
        get => (int)GetValue(IndexProperty);
        set => SetValue(IndexProperty, value);
    }
}

And the XAML code is something like this:

<local:IndexedTextBox Table="{Binding ViewModelTable}" Index="1"/>

Sometimes is works, other times it won’t.

I noticed that the 2 properties (Table and Index) are being initialized in random order, and whenever the Index is loaded before the Table the control won’t work as expected.

Is there a workaround to that? Can we control the order of the Initialization or there any other thing I should be doing.

I’d greatly appreciate a feedback

Thanks

>Solution :

whenever the Index is loaded before the Table the control won’t work as expected …

So change the implementation to make it work by for example invoking the callback for both properties and check the state of the control before doing anything, e.g.:

public class IndexedTextBox : TextBox
{
    public static readonly DependencyProperty TableProperty = DependencyProperty.Register(nameof(Table),
                    typeof(IEnumerable<Record>), typeof(IndexedTextBox), new PropertyMetadata(null, TableChangedCallback));

    public static readonly DependencyProperty IndexProperty = DependencyProperty.Register(nameof(Index),
                    typeof(int), typeof(IndexedTextBox), new PropertyMetadata(0, IndexChangedCallback));

    private static void TableChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e) =>
        ((IndexedTextBox)d).SetText();

    private static void IndexChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs e) =>
        ((IndexedTextBox)d).SetText();

    private void SetText()
    {
        if (Table != null)
        {
            int index = Index;

            if (Table.Any(t => t.Index == index))
                Text = Table.Where(t => t.Index == index).FirstOrDefault().Value;
            else
                Text = "N/A";
        }
    }

    public IEnumerable<Record> Table
    {
        get => (IEnumerable<Record>)GetValue(TableProperty);
        set => SetValue(TableProperty, value);
    }

    public int Index
    {
        get => (int)GetValue(IndexProperty);
        set => SetValue(IndexProperty, value);
    }
}