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

namespace TfsExcelImporter
{
    /// <summary>
    /// 输入框空态占位文字（cue banner）。net48 的 TextBox 没有 PlaceholderText，
    /// 用原生 EM_SETCUEBANNER 实现；ComboBox（DropDown）要发给它内嵌的 Edit 子窗口，
    /// 直接发给组合框本体无效。文字只在输入框为空时显示，与 Web 表单 placeholder 手感一致。
    /// </summary>
    internal static class CueBanner
    {
        private const int EM_SETCUEBANNER = 0x1501;
        private const uint GW_CHILD = 5;

        [DllImport("user32.dll", CharSet = CharSet.Unicode)]
        private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, string lParam);

        [DllImport("user32.dll")]
        private static extern IntPtr GetWindow(IntPtr hWnd, uint cmd);

        /// <summary>设置占位文字。控件句柄可能尚未创建，挂 HandleCreated 兜底；
        /// wParam=1 让占位文字在聚焦时也显示（有真实输入后自然消失）。</summary>
        public static void Set(Control c, string text)
        {
            if (c == null || string.IsNullOrEmpty(text)) return;
            Action apply = () => Send(c, text);
            if (c.IsHandleCreated) apply();
            c.HandleCreated += (s, e) => apply();
        }

        private static void Send(Control c, string text)
        {
            IntPtr target = c.Handle;
            if (c is ComboBox)
            {
                // DropDown 组合框的内嵌 Edit 子窗口：cue banner 只认它
                target = GetWindow(c.Handle, GW_CHILD);
                if (target == IntPtr.Zero) return;
            }
            SendMessage(target, EM_SETCUEBANNER, (IntPtr)1, text);
        }
    }
}