public class PasswordBox : TextBox
{
    #region Private Member Variables

    private string _text = string.Empty;
    private char _passwordChar = '\u25CF';

    #endregion

    #region Constructor

    /// <summary>
    /// Default Class Constructor
    /// </summary>
    public PasswordBox()
    {
        this.TextChanged += new TextChangedEventHandler(TextBox_TextChanged);
    }


    #endregion


    #region Private Methods


    private void TextBox_TextChanged(object sender, TextChangedEventArgs e)
    {
        TextBox myTxtBox = sender as TextBox;

        if (myTxtBox != null)
        {
            string currentText = myTxtBox.Text;
            int selStart = myTxtBox.SelectionStart;

            if (currentText.Length < _text.Length)
            {
                // Remove deleted chars
                _text = _text.Remove(selStart, _text.Length - currentText.Length);
            }
            if (currentText != "")
            {
                for (int i = 0; i < currentText.Length; i++)
                {
                    if (currentText[i] != _passwordChar)
                    {
                        // Unattach event handler
                        myTxtBox.TextChanged -= new TextChangedEventHandler(TextBox_TextChanged);
                        myTxtBox.Text = myTxtBox.Text.Remove(i, 1).Insert(i, _passwordChar.ToString());
                        myTxtBox.TextChanged += new TextChangedEventHandler(TextBox_TextChanged);
                        // Replace or insert char
                        _text = myTxtBox.Text.Length == _text.Length ?
                                _text.Remove(i, 1).Insert(i, currentText[i].ToString()) :
                                _text.Insert(i, currentText[i].ToString());
                    }
                }
            }
            myTxtBox.SelectionStart = selStart;
        }
    }

    #endregion
    #region Properties

    public new string Text
    {
        get { return _text; }
    }

    #endregion
}