[C#/WINFORM] TextBox 클래스 : 워터마크 문자열 표시하기

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

private const uint ECM_FIRST       = 0x1500;
private const uint EM_SETCUEBANNER = ECM_FIRST + 1;

/// <summary>
/// 메시지 보내기
/// </summary>
/// <param name="windowHandle">윈도우 핸들</param>
/// <param name="message">메시지</param>
/// <param name="wordParameter">WORD 파라미터</param>
/// <param name="longParameter">LONG 파라미터</param>
/// <returns>처리 결과</returns>
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
private static extern IntPtr SendMessage(IntPtr windowHandle, uint message, uint wordParameter, [MarshalAs(UnmanagedType.LPWStr)] string longParameter);

#region 워터마크 설정하기 - SetWatermark(textBox, watermarkText)

/// <summary>
/// 워터마크 설정하기
/// </summary>
/// <param name="textBox">텍스트 박스</param>
/// <param name="watermarkText">워터마크 텍스트</param>
public static void SetWatermark(this TextBox textBox, string watermarkText)
{
    SendMessage(textBox.Handle, EM_SETCUEBANNER, 0, watermarkText);
}

#endregion