Start Excel and get workbook handle

Question

Hi,
I would like to build a launcher which would start Excel 2013, and then register my xll file using "RegisterXLL".
One of my prototypes is starting the process, using the path to Excel.exe, and then it gets the window handle (the ultimate goal is to reach the COM application object in order to invoke RegisterXLL).
Thus I must find the child window whose name is "EXCEL7". I can get the main window handle from the process I have created, that's fine. But in order to get the child handle, I have tried 2 functions : EnumChildWindows and FindWindowExA,
which both are part of "user32.dll".
Unfortunately, I have to wait for a certain amount of time before calling these functions, otherwise they don't work. Basically, I wait for 1 or 2 seconds, then I call EnumChildWindows or FindWindowExA, and it works fine.
But if I don't wait, then EnumChildWindows does not enumerate anything, and FindWindowExA return 0. And once I have called one of them, I can retry even 10 seconds later, they will still not work.
Then I have no idea what I must wait for, or how long I must wait.
I would gratefully appreciate some help !
Thanks
Ron

Answer 1

Going back to my first approach using Windows apis, I noticed that if I refresh the process after a while, then I get a different value for the MainWindowHandle. From that value, I can do the work. 
Then here is my final solution :
1) Get the MainWindowHandle
2) Enumerate the child windows (using FindWindowExA or EnumChildWindows)
3) If the child window called "EXCEL7" is not found then refresh the process and go to 1). If the window was found then continue.
It works !
ProcessStartInfo info = new ProcessStartInfo(excelPath, "/x")
UseShellExecute = false,
CreateNoWindow = true
Process process = new Process
StartInfo = info
process.Start();
IntPtr hWndChild = IntPtr.Zero;
do
process.WaitForInputIdle();
StringBuilder cname = new StringBuilder(256);
EnumChildWindows(process.MainWindowHandle, delegate(IntPtr hWndEnum, IntPtr param)
GetClassNameW(hWndEnum, cname, cname.Capacity);
if (cname.ToString() == "EXCEL7")
hWndChild = hWndEnum;
return false;
return true;
}, IntPtr.Zero);
if (hWndChild == IntPtr.Zero)
Thread.Sleep(100);
process.Refresh();
} while (hWndChild == IntPtr.Zero);
IntPtr pUnk = IntPtr.Zero;
int hr = AccessibleObjectFromWindow(hWndChild, OBJID_NATIVEOM, IID_IDispatchBytes, ref pUnk);
if (hr >= 0)
dynamic obj = Marshal.GetObjectForIUnknown(pUnk);
dynamic app = obj.Application;
app.RegisterXLL(xllPath);