How to change the button text of another programs window

Question

I have been tasked with changing the text of a button in a window. I don't have and cannot access the source code as it's owned by a company we have a paid subscription with.

How can I change the button text with no source code? I'm trying with pInvoke and having problems. The window title changes depending on who you are working with:

"Order Entry Sheet - LASTNAME, FIRSTNAME"

So the window title may not be useable for me inside of the win32 call

FindWindow(string lpClassName, string lpWindowName);

I know both params are optional. I'm using Spy++ and I'm not sure what to use for lpClassName. The class name I see listed is #32770 (Dialog). I tried it and got a return of 0.

IntPtr windowHandle = FindWindow("#32770 (Dialog)", null);

How can I change the button text from another process?

UPDATE

According to MSDN I should be able to achieve this via SetWindowText.

Changes the text of the specified window's title bar (if it has one). If the specified window is a control, the text of the control is changed. However, SetWindowText cannot change the text of a control in another application.

I can't use SetWindowText to do what I want. Is it possible to use something else?

Answer 1

    [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);

    [DllImport("user32", SetLastError = true)]
    public static extern int EnumWindows(CallBack x, int y);

    [DllImport("user32.dll", SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    static extern bool EnumChildWindows(IntPtr hwndParent, CallBack lpEnumFunc, IntPtr lParam);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern IntPtr SendMessage(HandleRef hWnd, uint Msg, IntPtr wParam, string lParam);

    public const uint WM_SETTEXT = 0x000C;

    public delegate bool CallBack(int hwnd, int lParam);

    public static void Main()
    {
        CallBack windowsCallback = new CallBack(IterateWindows);
        EnumWindows(windowsCallback, 0);        
    }

    public static bool IterateChildren(int hwnd, int lParam)
    {
        string newButtonText = "Some text";
        bool continueIteratingChildren = true;
        //Console.WriteLine("Child handle: " + hwnd);

        int length = GetWindowTextLength((IntPtr)hwnd);
        StringBuilder sb = new StringBuilder(length + 1);
        GetWindowText((IntPtr)hwnd, sb, sb.Capacity);
        //Console.WriteLine(sb);

        if (sb.ToString().StartsWith("My Button Text "))
        {
            HandleRef hrefHWndTarget = new HandleRef(null, (IntPtr)hwnd);
            SendMessage(hrefHWndTarget, WM_SETTEXT, IntPtr.Zero, newButtonText);
            continueIteratingChildren = false;
        }
        return continueIteratingChildren;
    }

    public static bool IterateWindows(int hwnd, int lParam)
    {
        bool continueIteratingWindows = true;
        int windowTextLength = GetWindowTextLength((IntPtr)hwnd);
        StringBuilder sb = new StringBuilder(windowTextLength + 1);
        GetWindowText((IntPtr)hwnd, sb, sb.Capacity);

        if (sb.ToString().StartsWith("My Window Caption"))
        {
            //Console.Write("Window handle is ");
            //Console.WriteLine(hwnd);
            //Console.WriteLine(sb);
            //Console.WriteLine(Marshal.GetLastWin32Error());
            var childrenCallback = new CallBack(IterateChildren);
            EnumChildWindows((IntPtr)hwnd, childrenCallback, IntPtr.Zero);
            continueIteratingWindows = false;
        }
        return continueIteratingWindows;
    }