MessageBox

    private void validateUserEntry()
    {

        // Checks the value of the text.

        if(serverName.Text.Length == 0)
        {

            // Initializes the variables to pass to the MessageBox.Show method.

            string message = "You did not enter a server name. Cancel this operation?";
                        string caption = "Error Detected in Input";
            MessageBoxButtons buttons = MessageBoxButtons.YesNo;
            DialogResult result;

            // Displays the MessageBox.

            result = MessageBox.Show(message, caption, buttons);

            if (result == System.Windows.Forms.DialogResult.Yes)
            {

                // Closes the parent form.

                this.Close();

            }

        }

    }

기타 관련 자료
MessageBoxButtons 열거형
MessageBoxDefaultButton 열거형
MessageBoxIcon 열거형
MessageBoxOptions 열거형