private void textBox1_KeyPress(object sender, KeyPressEventArgs e)

        {           
            int sLength = textBox1.SelectionStart;  

            switch (sLength)
            {
                case 0:

                case 1:

                case 2:

                case 3:

                case 4:

                        if (char.IsLetter(e.KeyChar) || Char.IsControl(e.KeyChar))
                        {
                            e.Handled = false;
                        }
                        else
                        {
                            e.Handled = true;
                        }
                        break;

                case 5:

                case 6:

                case 7:

                case 8:
                        if (char.IsNumber(e.KeyChar) || Char.IsControl(e.KeyChar))
                        {
                            e.Handled = false;
                        }
                        else
                        {
                            e.Handled = true;
                        }
                        break;
                case 9:
                        if (char.IsLetter(e.KeyChar) || Char.IsControl(e.KeyChar))
                        {
                            e.Handled = false;
                        }
                        else
                        {
                            if (Char.IsControl(e.KeyChar))
                            {
                                e.Handled = false;
                            }

                            else
                            {
                                e.Handled = true;
                            }
                        }
                        break;
                default:
                        if (Char.IsControl(e.KeyChar))
                        {
                            e.Handled = false;
                        }

                        else
                        {
                            e.Handled = true;
                        }
                        break;
            }
        }