public class TimeTextBox : TextBox
    {
        //Check typed character and avoid illegal character.
        protected override void OnKeyPress(KeyPressEventArgs e)
        {
            base.OnKeyPress(e);
            //Save the old states.
            string txt = this.Text;
            int selectStart = this.SelectionStart;
            int selectLen = this.SelectionLength;
            if (Char.IsControl(e.KeyChar) == false)
            {
                //Cope with the common char, such as A,B,C,!,1,2
                txt = txt.Remove(selectStart, selectLen);
                txt = txt.Insert(selectStart, e.KeyChar.ToString());
                selectStart += 1;
            }
            else
            {
                //Cope with control key.
                Keys cmdKey = (Keys)e.KeyChar;
                if (cmdKey == Keys.Delete)
                {
                    if (selectLen == 0 && selectStart < txt.Length)
                        txt = txt.Remove(selectStart + 1);
                    else
                        txt = txt.Remove(selectStart, selectLen);
                }
                else if (cmdKey == Keys.Back)
                {
                    if (selectLen == 0 && selectStart > 0)
                    {
                        txt = txt.Remove(selectStart - 1);
                        selectStart -= 1;
                    }
                    else
                        txt = txt.Remove(selectStart, selectLen);
                }
            }

            if (ValidText(txt))
            {
                //Reset the text and states.
                this.Text = txt.ToUpper();
                this.SelectionStart = selectStart;
            }            
            e.Handled = true;
        }

        //Check if the typed character would generate a legal text
        private bool ValidText(string txt)
        {
            bool isLegal = true;
            for (int i = 0; i < txt.Length; i++)
            {                
                char c = txt[i];
                bool isCharLegal = false;
                switch (i)
                {
                    case 0:
                        {
                            //The first character must be 0 or 1
                            if (c == '0' || c == '1')
                                isCharLegal = true;
                            break;
                        }
                    case 1:
                        {
                            //The second character depends on the first character
                            //If the first is 0, this character can range from 1 to 9,
                            //else it must be 1 or 2
                            if((txt[0] == '0' && (c >= '1' && c <= '9'))
                                || ((txt[0] == '1') && (c == '1' || c == '2')))
                                isCharLegal = true;
                            break;
                        }
                    case 2:
                        {
                            //The third character must be ':'
                            if (c == ':')
                                isCharLegal = true;
                            break;
                        }
                    case 3:
                        {
                            //The fourth character must range from 0 to 5,
                            //because minites must range from 0 to 59
                            if (c >= '0' && c <= '5')
                                isCharLegal = true;
                            break;
                        }
                    case 4:
                        {
                            //The fifth character can be any digit.
                            if (Char.IsDigit(c))
                                isCharLegal = true;
                            break;
                        }
                    case 5:
                        {
                            //The sixth character must be a blank to split the time and AM/PM
                            if (Char.IsWhiteSpace(c))
                                isCharLegal = true;
                            break;
                        }
                    case 6:
                        {
                            //The seventh character can be A or P, a lower character is also legal.
                            if (c == 'A' || c == 'P'
                                || c == 'a' || c == 'p')
                                isCharLegal = true;
                            break;
                        }
                    case 7:
                        {
                            //The eighth character can be M, a lower character is also legal.
                            if (c == 'M' || c == 'm')
                                isCharLegal = true;
                            break;
                        }
                    default:
                        break;

                }
                if (isCharLegal == false)
                {
                    //If one character is illegal, the string is illegal.
                    isLegal = false;
                    break;
                }
            }
            return isLegal;
        }
    }