Evento Control.KeyPress

        // Boolean flag used to determine when a character other than a number is entered.
        private bool nonNumberEntered = false;

        // Handle the KeyDown event to determine the type of character entered into the control.
        private void textBox1_KeyDown(object sender, System.Windows.Forms.KeyEventArgs e)
        {
            // Initialize the flag to false.
            nonNumberEntered = false;

            // Determine whether the keystroke is a number from the top of the keyboard.
            if (e.KeyCode < Keys.D0 || e.KeyCode > Keys.D9)
            {
                // Determine whether the keystroke is a number from the keypad.
                if (e.KeyCode < Keys.NumPad0 || e.KeyCode > Keys.NumPad9)
                {
                    // Determine whether the keystroke is a backspace.
                    if(e.KeyCode != Keys.Back)
                    {
                        // A non-numerical keystroke was pressed.
                        // Set the flag to true and evaluate in KeyPress event.
                        nonNumberEntered = true;
                    }
                }
            }
            //If shift key was pressed, it's not a number.
            if (Control.ModifierKeys == Keys.Shift) {
                nonNumberEntered = true;
            }
        }

        // This event occurs after the KeyDown event and can be used to prevent
        // characters from entering the control.
        private void textBox1_KeyPress(object sender, System.Windows.Forms.KeyPressEventArgs e)
        {
            // Check for the flag being set in the KeyDown event.
            if (nonNumberEntered == true)
            {
                // Stop the character from being entered into the control since it is non-numerical.
                e.Handled = true;
            }
        }