MessageBox-Klasse

private void Form1_FormClosing(object sender, FormClosingEventArgs e)
{
    const string message =
        "Are you sure that you would like to close the form?";
    const string caption = "Form Closing";
    var result = MessageBox.Show(message, caption,
                                 MessageBoxButtons.YesNo,
                                 MessageBoxIcon.Question);

    // If the no button was pressed ...
    if (result == DialogResult.No)
    {
        // cancel the closure of the form.
        e.Cancel = true;
    }
}

	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();

			}

		}

	}