在C#中使用FindWindowEx函數，需要引入WindowsAPI的頭文件和函數聲明：

using System;
using System.Runtime.InteropServices;
public class WindowsAPI
{
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
[DllImport("user32.dll", SetLastError = true)]
public static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter, string lpszClass, string lpszWindow);
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern int GetWindowText(IntPtr hWnd, out string lpString, int nMaxCount);
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
public static extern int GetClassName(IntPtr hWnd, out string lpClassName, int nMaxCount);
}
public class Program
{
public static void Main()
{
string windowTitle = "Window Title";
IntPtr mainHWND = WindowsAPI.FindWindow(null, windowTitle);
if (mainHWND != IntPtr.Zero)
{
IntPtr childHWND = WindowsAPI.FindWindowEx(mainHWND, IntPtr.Zero, null, null);
if (childHWND != IntPtr.Zero)
{
string className;
WindowsAPI.GetClassName(childHWND, out className, 256);
string windowText;
WindowsAPI.GetWindowText(childHWND, out windowText, 256);
Console.WriteLine("Child HWND: " + childHWND.ToString());
Console.WriteLine("Class Name: " + className);
Console.WriteLine("Window Text: " + windowText);
}
else
{
Console.WriteLine("Child HWND not found.");
}
}
else
{
Console.WriteLine("Main HWND not found.");
}
}
}

在上述代碼中，首先通過FindWindow函數查找到指定標題的主窗口句柄mainHWND，然后通過FindWindowEx函數在主窗口下查找子窗口句柄childHWND。最后通過GetClassName和GetWindowText函數獲取子窗口的類名和窗口文本，并輸出到控制臺上。請注意，FindWindow和FindWindowEx函數的第一個參數lpClassName可以指定窗口類名，如果為null則表示忽略類名，只根據窗口標題查找。