How can I get functionality similar to Spy++ in my C# app?

I’ve being answering similar questions like this here: How can I detect if a thread has windows handles?. Like it states, the main idea is to enumerate through process windows and their child windows using EnumWindows and EnumChildWindows API calls to get window handles and then call GetWindowText or SendDlgItemMessage with WM_GETTEXT to get window text. I’ve modified code to make an example which should be doing what you need (sorry it’s a bit long :). It iterates through processes and their windows and dumps window text into console.

static void Main(string[] args)
{
    foreach (Process procesInfo in Process.GetProcesses())
    {
        Console.WriteLine("process {0} {1:x}", procesInfo.ProcessName, procesInfo.Id);
        foreach (ProcessThread threadInfo in procesInfo.Threads)
        {
            // uncomment to dump thread handles
            //Console.WriteLine("\tthread {0:x}", threadInfo.Id);
            IntPtr[] windows = GetWindowHandlesForThread(threadInfo.Id);
            if (windows != null && windows.Length > 0)
                foreach (IntPtr hWnd in windows)
                    Console.WriteLine("\twindow {0:x} text:{1} caption:{2}",
                        hWnd.ToInt32(), GetText(hWnd), GetEditText(hWnd));
        }
    }
    Console.ReadLine();
}

private static IntPtr[] GetWindowHandlesForThread(int threadHandle)
{
    _results.Clear();
    EnumWindows(WindowEnum, threadHandle);
    return _results.ToArray();
}

// enum windows

private delegate int EnumWindowsProc(IntPtr hwnd, int lParam);

[DllImport("user32.Dll")]
private static extern int EnumWindows(EnumWindowsProc x, int y);
[DllImport("user32")]
private static extern bool EnumChildWindows(IntPtr window, EnumWindowsProc callback, int lParam);
[DllImport("user32.dll")]
public static extern int GetWindowThreadProcessId(IntPtr handle, out int processId);

private static List<IntPtr> _results = new List<IntPtr>();

private static int WindowEnum(IntPtr hWnd, int lParam)
{
    int processID = 0;
    int threadID = GetWindowThreadProcessId(hWnd, out processID);
    if (threadID == lParam)
    {
        _results.Add(hWnd);
        EnumChildWindows(hWnd, WindowEnum, threadID);
    }
    return 1;
}

// get window text

[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
static extern int GetWindowText(IntPtr hWnd, StringBuilder lpString, int nMaxCount);
[DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern int GetWindowTextLength(IntPtr hWnd);

private static string GetText(IntPtr hWnd)
{
    int length = GetWindowTextLength(hWnd);
    StringBuilder sb = new StringBuilder(length + 1);
    GetWindowText(hWnd, sb, sb.Capacity);
    return sb.ToString();
}

// get richedit text 

public const int GWL_ID = -12;
public const int WM_GETTEXT = 0x000D;

[DllImport("User32.dll")]
public static extern int GetWindowLong(IntPtr hWnd, int index);
[DllImport("User32.dll")]
public static extern IntPtr SendDlgItemMessage(IntPtr hWnd, int IDDlgItem, int uMsg, int nMaxCount, StringBuilder lpString);
[DllImport("User32.dll")]
public static extern IntPtr GetParent(IntPtr hWnd);

private static StringBuilder GetEditText(IntPtr hWnd)
{
    Int32 dwID = GetWindowLong(hWnd, GWL_ID);
    IntPtr hWndParent = GetParent(hWnd);
    StringBuilder title = new StringBuilder(128);
    SendDlgItemMessage(hWndParent, dwID, WM_GETTEXT, 128, title);
    return title;
}

hope this helps, regards