[pax]

Собрал в один исходник

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        delegate bool EnumThreadDelegate(IntPtr hWnd, IntPtr lParam);

        [DllImport("user32.dll")]
        static extern bool EnumThreadWindows(int dwThreadId, EnumThreadDelegate lpfn,
            IntPtr lParam);

        static IEnumerable<IntPtr> EnumerateProcessWindowHandles(int processId)
        {
            var handles = new List<IntPtr>();

            foreach (ProcessThread thread in Process.GetProcessById(processId).Threads)
                EnumThreadWindows(thread.Id,
                    (hWnd, lParam) => { handles.Add(hWnd); return true; }, IntPtr.Zero);

            return handles;
        }

        private const uint WM_GETTEXT = 0x000D;

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, int wParam,
            StringBuilder lParam);


        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        public static extern int GetClassName(IntPtr hWnd,
           StringBuilder lpClassName,
           int nMaxCount
        );

        public static string GetWindowClassName(IntPtr hWnd)
        {
            StringBuilder buffer = new StringBuilder(128);
            GetClassName(hWnd, buffer, buffer.Capacity);
            return buffer.ToString().Trim();
        }


        public static string GetWindowName(IntPtr hWnd)
        {
            StringBuilder message = new StringBuilder(1000);
            SendMessage(hWnd, WM_GETTEXT, message.Capacity, message);
            return message.ToString().Trim();
        }


        public static IntPtr FindWindowByNameInProcess(string windowName, string processName)
        {
            var processWindowHandles = EnumerateProcessWindowHandles(
                Process.GetProcessesByName("explorer").First().Id);

            foreach (var handle in processWindowHandles)
            {
                if (GetWindowName(handle).StartsWith(windowName))
                {
                    return handle;
                }
            }

            return IntPtr.Zero;
        }

        public static IntPtr FindWindowByClassInProcess(string windowClassName, string processName)
        {
            var processWindowHandles = EnumerateProcessWindowHandles(
                Process.GetProcessesByName("explorer").First().Id);

            foreach (var handle in processWindowHandles)
            {
                if (GetWindowClassName(handle).StartsWith(windowClassName))
                {
                    return handle;
                }
            }

            return IntPtr.Zero;
        }

        [STAThread]
        static void Main(string[] args)
        {
            foreach (var handle in EnumerateProcessWindowHandles(
                Process.GetProcessesByName("explorer").First().Id))
            {
                Console.WriteLine("Name: "+GetWindowName(handle));
                Console.WriteLine("Class: " + GetWindowClassName(handle));
            }

            // поиск по имени класса
            var hwnd = FindWindowByClassInProcess("tool", "explorer");
            if (hwnd != IntPtr.Zero)
            {
                Console.WriteLine("Finded Name: " + GetWindowName(hwnd));
                Console.WriteLine("Finded Class: " + GetWindowClassName(hwnd));
            }

            // поиск по имени окна
            hwnd = FindWindowByNameInProcess("Перек", "explorer");
            if (hwnd != IntPtr.Zero)
            {
                Console.WriteLine("Finded Name: " + GetWindowName(hwnd));
                Console.WriteLine("Finded Class: " + GetWindowClassName(hwnd));
            }

            Console.ReadLine();


        }
    }
}