Нажатие на кнопку в ToolBar (SendMessage) в стороннем приложении(!Не в InternetExplorer!)

 [DllImport("USER32.DLL", CharSet = CharSet.Unicode)]
    public static extern IntPtr FindWindow(string lpClassName,      string lpWindowName);
    [DllImport("USER32.DLL")]
    static extern IntPtr FindWindowEx(IntPtr parentHandle, IntPtr childAfter, string lclassName, string windowTitle);
    
    [DllImport("user32.dll")]
    public static extern int GetWindowThreadProcessId(IntPtr handle, out int processId);
    [DllImport("kernel32.dll")]
    public static extern IntPtr OpenProcess(
         ProcessAccessFlags processAccess,
         bool bInheritHandle,
         int processId
    );
    [DllImport("user32.dll")]
    public static extern int SendMessage(IntPtr hWnd, int wMsg, IntPtr wParam, IntPtr lParam);
    [DllImport("user32.dll", SetLastError = true)]
    static extern bool PostMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);
    [DllImport("kernel32.dll", SetLastError = true)]
    static extern Int32 ReadProcessMemory(IntPtr hProcess, IntPtr lpBaseAddress, [Out] byte[] buffer, UInt32 size, out IntPtr lpNumberOfBytesRead);
    [DllImport("kernel32.dll", SetLastError = true, ExactSpelling = true)]
    static extern IntPtr VirtualAllocEx(IntPtr hProcess, IntPtr lpAddress,
       uint dwSize, AllocationType flAllocationType, MemoryProtection flProtect);
    [DllImport("kernel32.dll")]
    public static extern bool VirtualFreeEx(IntPtr hProcess, IntPtr lpAddress, uint dwSize, uint dwFreeType);
    [DllImport("kernel32.dll")]
    public static extern bool CloseHandle(IntPtr handle);

    [DllImport("user32.dll", SetLastError = true)]
    static extern bool GetWindowRect(IntPtr hwnd, out RECT lpRect);

    
    [Flags]
    public enum MouseEventFlags : uint
    {
        LEFTDOWN = 0x00000002,
        LEFTUP = 0x00000004,
        MIDDLEDOWN = 0x00000020,
        MIDDLEUP = 0x00000040,
        MOVE = 0x00000001,
        ABSOLUTE = 0x00008000,
        RIGHTDOWN = 0x00000008,
        RIGHTUP = 0x00000010
    }
    public enum VirtualFreeExTypes : uint
    {
        MEM_DECOMMIT = 0x4000,
        MEM_RELEASE = 0x8000
    }

    [Flags]
    public enum AllocationType
    {
        Commit = 0x1000,
        Reserve = 0x2000,
        Decommit = 0x4000,
        Release = 0x8000,
        Reset = 0x80000,
        Physical = 0x400000,
        TopDown = 0x100000,
        WriteWatch = 0x200000,
        LargePages = 0x20000000
    }
    [Flags]
    public enum MemoryProtection
    {
        Execute = 0x10,
        ExecuteRead = 0x20,
        ExecuteReadWrite = 0x40,
        ExecuteWriteCopy = 0x80,
        NoAccess = 0x01,
        ReadOnly = 0x02,
        ReadWrite = 0x04,
        WriteCopy = 0x08,
        GuardModifierflag = 0x100,
        NoCacheModifierflag = 0x200,
        WriteCombineModifierflag = 0x400
    }
    [Flags]
    public enum ProcessAccessFlags : uint
    {
        All = 0x001F0FFF,
        Terminate = 0x00000001,
        CreateThread = 0x00000002,
        VirtualMemoryOperation = 0x00000008,
        VirtualMemoryRead = 0x00000010,
        VirtualMemoryWrite = 0x00000020,
        DuplicateHandle = 0x00000040,
        CreateProcess = 0x000000080,
        SetQuota = 0x00000100,
        SetInformation = 0x00000200,
        QueryInformation = 0x00000400,
        QueryLimitedInformation = 0x00001000,
        Synchronize = 0x00100000
    }
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    {
        public int Left, Top, Right, Bottom;
    }
    const int WM_USER = 0x0400;
    const int TB_GETITEMRECT = (WM_USER + 29);
    const int WM_LBUTTONDOWN = 0x0201;
    const int WM_LBUTTONUP = 0x0202;
    public static int MAKELONG(int x, int y)
    {
        return (x & 0xffff) | ((y & 0xffff) << 16);
    }
    public static RECT ByteArrayToRect(byte[] bytes)
    {
        GCHandle handle = GCHandle.Alloc(bytes, GCHandleType.Pinned);
        RECT stuff = (RECT)Marshal.PtrToStructure(
            handle.AddrOfPinnedObject(), typeof(RECT));
        handle.Free();
        return stuff;
    }
    private void button1_Click(object sender, EventArgs e)
    {
       
        IntPtr form = FindWindow("ThunderRT6FormDC", "Form1");
        IntPtr toolbar = FindWindowEx(form, IntPtr.Zero, "Toolbar20WndClass", string.Empty);
       IntPtr msvb_tool = FindWindowEx(toolbar, IntPtr.Zero, "msvb_lib_toolbar", string.Empty);

        IntPtr nNumberOfBytesRead;

        RECT winRect = new RECT();
        GetWindowRect(msvb_tool, out winRect);

        int dwProcessID;
        GetWindowThreadProcessId(msvb_tool, out dwProcessID);
        
        IntPtr hProcess = OpenProcess((ProcessAccessFlags.VirtualMemoryOperation | ProcessAccessFlags.VirtualMemoryRead | ProcessAccessFlags.VirtualMemoryWrite), false, dwProcessID);// prc.Handle;//Process.GetCurrentProcess(); //
        IntPtr Pointer = VirtualAllocEx(hProcess, (IntPtr)0, 4096, AllocationType.Reserve | AllocationType.Commit, MemoryProtection.ReadWrite);

        RECT bi;
        SendMessage(msvb_tool, TB_GETITEMRECT, (IntPtr)0 /* индекс кнопки*/, (IntPtr)Pointer);
        byte[] buffer = new byte[16];
        ReadProcessMemory(hProcess, (Pointer), buffer, (uint)buffer.Length, out nNumberOfBytesRead);
        bi = ByteArrayToRect(buffer);
        PostMessage(msvb_tool, WM_LBUTTONDOWN, (IntPtr)0, (IntPtr)MAKELONG(bi.Left, bi.Top));
        PostMessage(msvb_tool, WM_LBUTTONUP, (IntPtr)0, (IntPtr)MAKELONG(bi.Left, bi.Top));
       
        VirtualFreeEx(hProcess, Pointer, 0, (uint)VirtualFreeExTypes.MEM_RELEASE);
        CloseHandle(hProcess);
    }
}

Вот рабочий код! Пользуйтесь! Помог мне разобраться WinSpy, указав на матрешку контролов.