Textbox with placeholder by C#

It's here. A textbox with placeholder. Look like input control in html. Create a file named STextBox.cs and paste this code:

// -----------------------------------------------------------------------------
// File name: STextBox.cs
// Author: Smart Goat
// Email: dmtmd2010@yahoo.com.vn
// Blog: knowledgesharez.net
// Date: 2013-12-09
// Description: a text box with placeholder, similar the web input control.
//   - Properties:
//      + Placeholder: string - The content of the placeholder
//   - Methods:
//      + IsEmpty(): bool - Check if the text box is empty (recommended)
// -----------------------------------------------------------------------------

using System;
using System.Drawing;
using System.Windows.Forms;

namespace FooBar
{
    class STextBox : TextBox
    {
        private bool textInputted;
        private Color backupForeColor = Color.Empty;
        private string placeholder = string.Empty;

        public string Placeholder
        {
            get { return placeholder; }
            set
            {
                placeholder = value;
                SetPlaceHolder();
            }
        }

        public bool IsEmpty()
        {
            return string.IsNullOrEmpty(Text) || !textInputted;
        }

        private void SetPlaceHolder()
        {
            if (string.IsNullOrEmpty(Text))
            {
                Text = placeholder;
                ForeColor = Color.Gray;
                textInputted = false;
            }
        }

        protected override void OnTextChanged(EventArgs e)
        {
            base.OnTextChanged(e);
            textInputted = !string.IsNullOrEmpty(Text);
        }

        protected override void OnEnter(EventArgs e)
        {
            base.OnEnter(e);
            if (!textInputted)
            {
                Text = string.Empty;
                ForeColor = backupForeColor;
            }
        }

        protected override void OnLeave(EventArgs e)
        {
            base.OnLeave(e);
            SetPlaceHolder();
        }

        protected override void OnForeColorChanged(EventArgs e)
        {
            base.OnForeColorChanged(e);
            if (backupForeColor == Color.Empty)
            {
                backupForeColor = ForeColor;
            }
        }

        protected override void OnKeyDown(KeyEventArgs e)
        {
            base.OnKeyDown(e);
            if (e.Control && e.KeyCode == Keys.A)
            {
                SelectAll();
            }
        }
    }
}