// First lets create a var that checks if the input is valid
private bool validInput = 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.
	validInput  = false;

	// We are checking if it is not that valid button pressed
	if (
		e.KeyCode < Keys.D0 || e.KeyCode > Keys.D9 ||
		e.KeyCode < Keys.NumPad0 ||  e.KeyCode > Keys.NumPad9 ||   
  		e.KeyCode != Keys.Oemplus || e.KeyCode != OemPeriod
	){
                validInput = 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 (validInput == true)
    {
        // Stop the character from being entered into the control since it is non-numerical.
        e.Handled = true;
        // reset bool for next keydown event validation.
        validInput = false;
    }
}