Input.cs

    [TemplatePart(Name = "Part_Clear", Type = typeof(Border))]
    [TemplatePart(Name = "Part_TxtBox", Type = typeof(TextBox))]
    [TemplatePart(Name = "Part_Placeholder", Type = typeof(TextBlock))]
    public class Input : TextBox
    {
        private Border _clearIcon;
        private TextBox _txtBox;
        private TextBlock _txtBlockPlaceholder;
        static Input()
        {
            DefaultStyleKeyProperty.OverrideMetadata(typeof(Input), new FrameworkPropertyMetadata(typeof(Input)));
        }
        public Geometry Icon
        {
            get { return (Geometry)GetValue(IconProperty); } set { SetValue(IconProperty, value); }
        }
        public static readonly DependencyProperty IconProperty = DependencyProperty.Register("Icon", typeof(Geometry), typeof(Input), new PropertyMetadata(null));
        public string Placeholder
        {
            get { return (string)GetValue(PlaceholderProperty); } set { SetValue(PlaceholderProperty, value); }
        }
        public static readonly DependencyProperty PlaceholderProperty = DependencyProperty.Register("Placeholder", typeof(string), typeof(Input), new PropertyMetadata("PLACEHOLDER"));
        public string TextContent
        {
            get { return (string)GetValue(TextContentProperty); } set { SetValue(TextContentProperty, value); }
        }
        public static readonly DependencyProperty TextContentProperty = DependencyProperty.Register("TextContent", typeof(string), typeof(Input), new PropertyMetadata(default(string)));
        public override void OnApplyTemplate()
        {
            base.OnApplyTemplate();
            if (this.Template != null)
            {
                _clearIcon = this.Template.FindName("Part_Clear", this) as Border;
                _txtBox = this.Template.FindName("Part_TxtBox", this) as TextBox;
                _txtBlockPlaceholder = this.Template.FindName("Part_Placeholder", this) as TextBlock;
                if (_clearIcon != null) { _clearIcon.MouseLeftButtonDown += ClearIcon_MouseLeftButtonDown; }
                if (_txtBox != null) { _txtBox.TextChanged += TextBox_TextChanged; }
            }
        }
        private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            if (string.IsNullOrEmpty(_txtBox.Text))
            {
                _txtBlockPlaceholder.Visibility = Visibility.Visible;
            }
            else
            {
                _txtBlockPlaceholder.Visibility = Visibility.Hidden;
            }
        }
    private void ClearIcon_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    { 
        _txtBox.Clear();
        _txtBox.Focus(); }
    }