C# - Prevent User from entering specific characters into textbox.

// 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;
            }
        }
    }
}
 
// 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;
    }
}
                                              

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private void TextBox_KeyPress(object sender, KeyPressEventArgs e)
        {
            if (tbBlastMonitorName.Focused)
            {
                if (!(Char.IsLetter(e.KeyChar) || Char.IsDigit(e.KeyChar) || Char.IsControl(e.KeyChar)))
                    e.Handled = true;
            }
            else if (tbBlastMonitorIP.Focused)
            {
                if (!(Char.IsDigit(e.KeyChar) || Char.IsControl(e.KeyChar)))
                    e.Handled = true;
            }
            else if (tbProbeSettleTime.Focused)
            {
                if (!(Char.IsDigit(e.KeyChar) || Char.IsControl(e.KeyChar)))
                    e.Handled = true;
            }
        }

                                              

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