Win32.cs

using System;
using System.Runtime.InteropServices;

namespace L4D2DevTools
{
    public static class Win32
    {
        [DllImport("user32.dll")]
        private extern static IntPtr FindWindow(string lpClassName, string lpWindowName);

        [DllImport("user32.dll")]
        private static extern int SetForegroundWindow(IntPtr hwnd);

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

        [DllImport("user32.dll")]
        private static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);

        [DllImport("user32.dll")]
        private static extern int SendMessage(IntPtr hwnd, int wMsg, int wParam, int lParam);

        private enum GetWindowType : uint
        {
            GW_HWNDFIRST = 0,
            GW_HWNDLAST = 1,
            GW_HWNDNEXT = 2,
            GW_HWNDPREV = 3,
            GW_OWNER = 4,
            GW_CHILD = 5,
            GW_ENABLEDPOPUP = 6
        }

        private const int CB_SHOWDROPDOWN = 0x014F;
        private const int CB_SETCURSEL = 0x014E;
        private const int WM_SETFOCUS = 0x07;

        /// <summary>
        /// 设置区域MFC窗口Combobox控件选中索引
        /// </summary>
        /// <param name="index">0，501</param>
        public static void SetComboboxSelectIndex(int index)
        {
            var regionHandle = FindWindow(null, "区域");
            if (regionHandle != IntPtr.Zero)
            {
                SetForegroundWindow(regionHandle);

                var childHandle = GetWindow(regionHandle, GetWindowType.GW_CHILD);
                var comboboxHandle = FindWindowEx(childHandle, IntPtr.Zero, "ComboBox", null);

                SendMessage(comboboxHandle, CB_SHOWDROPDOWN, 1, 0);
                SendMessage(comboboxHandle, CB_SETCURSEL, index, 0);
                SendMessage(comboboxHandle, WM_SETFOCUS, 0, 0);
            }
        }
    }
}