好用的PasswordTextBox.

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace _0604_pwdtxb
{
    public class PasswordTextBox : TextBox
    {
        private readonly Timer timer;
        private Char[] adminPassword = new Char[16];
        private readonly char DecimalSeparator = CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator.ToCharArray()[0];

        /// <summary>
        /// 
        /// </summary>
        public PasswordTextBox()
        {
            timer = new Timer { Interval = 200 };
            timer.Tick += timer_Tick;
        }

        /// <summary>
        /// 
        /// </summary>
        public string AdminPassword
        {
            get
            {
                return new string(adminPassword).Trim('').Replace("", "");
            }
        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="e"></param>
        protected override void OnTextChanged(EventArgs e)
        {
            base.OnTextChanged(e);
            txtInput_TextChanged(this, e);

        }

        /// <summary>
        /// 
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void txtInput_TextChanged(object sender, EventArgs e)
        {
            HidePasswordCharacters();
        }

        /// <summary>
        /// 
        /// </summary>
        private void HidePasswordCharacters()
        {
            int index = this.SelectionStart;
            if (index > 1)
            {
                StringBuilder s = new StringBuilder(this.Text);
                s[index - 2] = '*';
                this.Text = s.ToString();
                this.SelectionStart = index;
            }
            timer.Enabled = true;
        }

        protected override void OnKeyDown(KeyEventArgs e)
        {
            base.OnKeyDown(e);
            if (e.KeyCode == Keys.Delete)
            {
                DeleteSelectedCharacters(this, e.KeyCode);
            }
        }

        /// <summary>
        /// Windows Timer elapsed eventhandler 
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void timer_Tick(object sender, EventArgs e)
        {
            timer.Enabled = false;
            int index = this.SelectionStart;
            if (index > 0)
            {
                StringBuilder s = new StringBuilder(this.Text);
                s[index-1] = '*';
                this.Invoke(new Action(() =>
                {
                    this.Text = s.ToString();
                    this.SelectionStart = index;
                }));
            }
        }

        protected override void OnKeyPress(KeyPressEventArgs e)
        {
            base.OnKeyPress(e);

            int selectionStart = this.SelectionStart;
            int length = this.TextLength;
            int selectedChars = this.SelectionLength;
            this.TextChanged -= new System.EventHandler(txtInput_TextChanged);

            if (selectedChars == length)
            {
                /*
                 * Means complete text selected so clear it before using it
                 */
                ClearCharBufferPlusTextBox();
            }

            Keys eModified = (Keys)e.KeyChar;

            if (e.KeyChar == DecimalSeparator)
            {
                e.Handled = true;
            }
            if ((Keys.Delete != eModified) && (Keys.Back != eModified))
            {
                if (Keys.Space != eModified)
                {
                    if (e.KeyChar != '-')
                    {
                        if (!char.IsLetterOrDigit(e.KeyChar))
                        {
                            e.Handled = true;
                        }
                        else
                        {
                            adminPassword = new string(adminPassword).Insert(selectionStart, e.KeyChar.ToString()).ToCharArray();
                        }
                    }
                }
                else
                {
                    if (this.TextLength == 0)
                    {
                        e.Handled = true;
                        Array.Clear(adminPassword, 0, adminPassword.Length);
                    }
                }
            }
            else if ((Keys.Back == eModified) || (Keys.Delete == eModified))
            {
                DeleteSelectedCharacters(this, eModified);
            }

            /*
             * Replace the characters with '*'
             */
            HidePasswordCharacters();

            this.TextChanged += new System.EventHandler(this.txtInput_TextChanged);
        }

        /// <summary>
        /// Deletes the specific characters in the char array based on the key press action
        /// </summary>
        /// <param name="sender"></param>
        private void DeleteSelectedCharacters(object sender, Keys key)
        {
            int selectionStart = this.SelectionStart;
            int length = this.TextLength;
            int selectedChars = this.SelectionLength;

            if (selectedChars == length)
            {
                ClearCharBufferPlusTextBox();
                this.TextChanged += new System.EventHandler(this.txtInput_TextChanged);
                return;
            }

            if (selectedChars > 0)
            {
                int i = selectionStart;
                this.Text.Remove(selectionStart, selectedChars);
                adminPassword = new string(adminPassword).Remove(selectionStart, selectedChars).ToCharArray();
            }
            else
            {
                /*
                 * Basically this portion of code is to handle the condition 
                 * when the cursor is placed at the start or in the end 
                 */
                if (selectionStart == 0)
                {
                    /*
                    * Cursor in the beginning, before the first character 
                    * Delete the character only when Del is pressed, No action when Back key is pressed
                    */
                    if (key == Keys.Delete)
                    {
                        adminPassword = new string(adminPassword).Remove(0, 1).ToCharArray();
                    }
                }
                else if (selectionStart > 0 && selectionStart < length)
                {
                    /*
                    * Cursor position anywhere in between 
                    * Backspace and Delete have the same effect
                    */
                    if (key == Keys.Back || key == Keys.Delete)
                    {
                        adminPassword = new string(adminPassword).Remove(selectionStart, 1).ToCharArray();
                    }
                }
                else if (selectionStart == length)
                {
                    /*
                    * Cursor at the end, after the last character 
                    * Delete the character only when Back key is pressed, No action when Delete key is pressed
                    */
                    if (key == Keys.Back)
                    {
                        adminPassword = new string(adminPassword).Remove(selectionStart - 1, 1).ToCharArray();
                    }
                }
            }
        }

        private void ClearCharBufferPlusTextBox()
        {
            Array.Clear(adminPassword, 0, adminPassword.Length);
            this.Clear();
        }
    }

}