How to access Microsoft Word existing instance using late binding

Question

i am developing some code in c# where i will be interacting with Microsoft Word. I want to be able to have the option of re-using an existing instance or as an alternative creating a new instance.

Keeping in mind i want to do all of this using LATE BINDING... it is safe to say i have figured out how to get things working when creating a new instance.. i just call Activator.CreateInstance etc..

The problem i am having is how do i reuse an existing instance, for example, Word is already open and i want to use that instance.

Is there an Activator.UseExistingInstance? or something similar??

Thanks!

Answer 1

You're looking for Marshal.GetActiveObject.

object word;
try
{
    word = System.Runtime.InteropServices.Marshal.GetActiveObject("Word.Application");
}
catch (COMException)
{
    Type type = Type.GetTypeFromProgID("Word.Application");
    word = System.Activator.CreateInstance(type);
}

Answer 2

You might want to have a look at the AccessibleObjectFromWindow api function defined in Oleacc.dll. Andrew Whitechapel has some articles on how to use it. Based on his articles I wrote an answer to a very similar question (about Excel, not Word), which you can find here:

How to use use late binding to get Excel instance?

There you will find an example how to connect to an already running Excel instance and then automating this instance using late binding.

Update:

Here is a short sample adapted to Word:

using System;
using System.Reflection;
using System.Runtime.InteropServices;
using System.Text;

namespace WordLateBindingSample
{
    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("00020400-0000-0000-C000-000000000046")]
    public interface IDispatch
    {
    }

    class Program
    {
        [DllImport("user32.dll", SetLastError = true)]
        static extern IntPtr FindWindow(string lpClassName, string lpWindowName);

        [DllImport("Oleacc.dll")]
        static extern int AccessibleObjectFromWindow(int hwnd, uint dwObjectID, byte[] riid, out IDispatch ptr);

        public delegate bool EnumChildCallback(int hwnd, ref int lParam);

        [DllImport("User32.dll")]
        public static extern bool EnumChildWindows(int hWndParent, EnumChildCallback lpEnumFunc, ref int lParam);

        [DllImport("User32.dll")]
        public static extern int GetClassName(int hWnd, StringBuilder lpClassName, int nMaxCount);

        public static bool EnumChildProc(int hwndChild, ref int lParam)
        {
            StringBuilder buf = new StringBuilder(128);
            GetClassName(hwndChild, buf, 128);
            if (buf.ToString() == "_WwG")
            {
                lParam = hwndChild;
                return false;
            }
            return true;
        }

        static void Main(string[] args)
        {
            // Use the window class name ("OpusApp") to retrieve a handle to Word's main window.
            // Alternatively you can get the window handle via the process id:
            // int hwnd = (int)Process.GetProcessById(wordPid).MainWindowHandle;
            //
            int hwnd = (int)FindWindow("OpusApp", null);

            if (hwnd != 0)
            {
                int hwndChild = 0;

                // Search the accessible child window (it has class name "_WwG") 
                // as described in http://msdn.microsoft.com/en-us/library/dd317978%28VS.85%29.aspx
                //
                EnumChildCallback cb = new EnumChildCallback(EnumChildProc);
                EnumChildWindows(hwnd, cb, ref hwndChild);

                if (hwndChild != 0)
                {
                    // We call AccessibleObjectFromWindow, passing the constant OBJID_NATIVEOM (defined in winuser.h) 
                    // and IID_IDispatch - we want an IDispatch pointer into the native object model.
                    //
                    const uint OBJID_NATIVEOM = 0xFFFFFFF0;
                    Guid IID_IDispatch = new Guid("{00020400-0000-0000-C000-000000000046}");
                    IDispatch ptr;

                    int hr = AccessibleObjectFromWindow(hwndChild, OBJID_NATIVEOM, IID_IDispatch.ToByteArray(), out ptr);

                    if (hr >= 0)
                    {
                        object wordApp = ptr.GetType().InvokeMember("Application", BindingFlags.GetProperty, null, ptr, null);

                        object version = wordApp.GetType().InvokeMember("Version", BindingFlags.GetField | BindingFlags.InvokeMethod | BindingFlags.GetProperty, null, wordApp, null);
                        Console.WriteLine(string.Format("Word version is: {0}", version));
                    }
                }
            }
        }
    }
}