Add ComboBoxAssist.MaxLength attached property in ComboBox to control MaxLength when IsEditable is set to True

[jamesport079]

ComboBox with IsEditable set to true essentially behaves as a TextBox with an autosuggest dropdown list. A silly ommission in WPF is that MaxLength cannot be set in XAML for an editable ComboBox.

I created an attached property to address this shortcoming. I strongly feel this can be integrated in this library in the form of materialDesign:ComboBoxAssist.MaxLength="___" helper class.

The code to implement it is pretty straightforward, essentially creating an attached property MaxLength and finding the TextBox part in ComboBox and setting MaxLength on TextBox. FindChild does the magic to find the TextBox part of the editable ComboxBox.

private static void OnMaxLenghtChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
{
    if (obj is ComboBox comboBox)
    {
        comboBox.Loaded +=
          (s, e) =>
          {
              DependencyObject? textBox = comboBox.FindChild<TextBox>("PART_EditableTextBox");
              if (textBox == null)
              {
                  return;
              }

              textBox.SetValue(TextBox.MaxLengthProperty, args.NewValue);
          };
    }
}

If you think this is beyond the scope of this library I would at least suggest to add TextFieldAssist.CharacterCounterVisibility to be accessible in an editable ComboBox for the sake of consistency.

[ElieTaillard]

FindChild does not exist in the current context. Do you mean FindName ?

[jamesport079]

sorry, here's the code for FindChild...

public static class UIChildFinder
{
    /// <summary>
    /// Finds a Child of a given item in the visual tree. 
    /// </summary>
    /// <param name="parent">A direct parent of the queried item.</param>
    /// <typeparam name="T">The type of the queried item.</typeparam>
    /// <param name="childName">x:Name or Name of child. </param>
    /// <returns>The first parent item that matches the submitted type parameter. 
    /// If not matching item can be found, 
    /// a null parent is being returned.</returns>
    public static T? FindChild<T>(this DependencyObject parent, string childName)
       where T : DependencyObject
    {
        // Confirm parent and childName are valid. 
        if (parent == null) return null;

        T? foundChild = null;

        int childrenCount = VisualTreeHelper.GetChildrenCount(parent);
        for (int i = 0; i < childrenCount; i++)
        {
            var child = VisualTreeHelper.GetChild(parent, i);
            // If the child is not of the request child type child
            if (child is not T)
            {
                // recursively drill down the tree
                foundChild = FindChild<T>(child, childName);

                // If the child is found, break so we do not overwrite the found child. 
                if (foundChild != null) break;
            }
            else if (!string.IsNullOrEmpty(childName))
            {
                // If the child's name is set for search
                if (child is FrameworkElement frameworkElement && frameworkElement.Name == childName)
                {
                    // if the child's name is of the request name
                    foundChild = (T)child;
                    break;
                }
            }
            else
            {
                // child element found.
                foundChild = (T)child;
                break;
            }
        }
        return foundChild;
    }
}

[ElieTaillard]

I made a pull request with the changes you suggested : #2662

[jamesport079]

Thank you 👍