How to set maxlength for combobox in WPF?-Collection of common programming errors

How do i set maxlength to combobox, which is having a style applied to it.

Thanks

Hey, using DependencyProperty, we can set the maxlength of the combo box without modifying your style/template.

public class EditableComboBox
{

    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(EditableComboBox), new UIPropertyMetadata(OnMaxLenghtChanged));

    private static void OnMaxLenghtChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
    {
        var comboBox = obj as ComboBox;
        if (comboBox == null) return;

        comboBox.Loaded +=
            (s, e) =>
            {
                var textBox = comboBox.FindChild(typeof(TextBox), "PART_EditableTextBox");
                if (textBox == null) return;

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

Usage example:

Where ComboboxHelper is:

xmlns:ComboboxHelper=”clr-namespace:yourNameSpace;assembly=yourAssembly“

comboBox.FindChild(…) method is posted here.

You are correct. There is a maxlength for a Textbox, but not for a combobox. You have to roll your own using a Textbox as an intermediary. Here’s some code:

public int MaxLength {get; set;}
protected override void OnGotFocus(System.Windows.RoutedEventArgs e)
{
    base.OnGotFocus(e);
    TextBox thisTextBox = (TextBox)base.GetTemplateChild("PART_EditableTextBox");
    if (thisTextBox != null)
        thisTextBox.MaxLength = MaxLength;
}

Or you can use the GotFocus or Loaded event of the combobox for setting the maxlength.If the maxlength doest change too much during runtime you can use loaded event or else use gotfocus event

and in the respective event…

   var obj = (ComboBox)sender;
    if (obj != null)
    {
        var myTextBox = (TextBox)obj.Template.FindName("PART_EditableTextBox",obj);
        if (myTextBox != null)
        {
            myTextBox.MaxLength = maxLength;
        }
    }

This TextBox is coming null. The style given is as follows.

        
            
                
                
            
            

            

            
        
        
            
                
            
            
                
            
            
                
                
                
                
            
        
    

    
        
    


    
        
        
        
        
        
        
        
        
            
                
                    
                        

                        

                        

                        

                            

                                

                                
                                    
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I found easy solution via XAML. In comboBox resources we can set style for textbox and via setter set maxlenth.