How to use use late binding to get excel instance?

Question

I'm using

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

to get an Excel Instance using his handle, which I get from the process ID of the excel instance.

This is how it looks like when I use these function

const uint OBJID_NATIVEOM = 0xFFFFFFF0;
Guid IID_IDispatch = new Guid("{00020400-0000-0000-C000-000000000046}");
Excel.Window ptr = null;  
int hr = AccessibleObjectFromWindow(hwndChild, OBJID_NATIVEOM, 
          IID_IDispatch.ToByteArray(), ref ptr);

Object objApp = ptr.Application;

This peace of code works great but the only problem is that I had to add a reference to the Office 2003 Primary Interop Assemblies.

As you can see, the last param in the function is the reason why I needed to add the reference to the Pias, so my question is if there is a way of avoiding the use of Interop Assemblies, I have tried with late binding but perhaps I've been doing it wrong because I haven't been able to make it work.

Answer 1

First: Late binding in C# is quite a pain. It's best to avoid it. Second: Late binding in C# is a pain. Use the PIA!

Ok, that being said, here is what you need to do in order to use late binding: Remove the reference to the Office 2003 PIAs and instead add a COM import of the interface required by AccessibleObjectFromWindow, i.e. the Excel.Window interface:

[Guid("00020893-0000-0000-C000-000000000046")]
[InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
public interface ExcelWindow
{
}

You can retrieve this interface using a tool like Reflector (or by simply pressing F12 on the type Excel.Window while the reference to the Excel PIA is still in your project)

That being done you will have to modify the signature of AccessibleObjectFromWindow to match the imported ExcelWindow interface:

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

Finally, you must use reflection to get the Excel.Application object from the ExcelWindow object:

object xlApp = ptr.GetType().InvokeMember("Application", BindingFlags.GetProperty, null, ptr, null);

If your code is going to make a lot of calls into Excel's OM it might be easier to use VB with Option Strict turned off (or wait for C#4.0 ;-). Or, if you don't want to change from C#, it might be a good idea to create a wrapper class for the late binding calls.

---

Full Sample

Here is a fully functional sample (based on an article by Andrew Whitechapel):

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

namespace ExcelLateBindingSample
{
    /// <summary>
    /// Interface definition for Excel.Window interface
    /// </summary>
    [Guid("00020893-0000-0000-C000-000000000046")]
    [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)]
    public interface ExcelWindow
    {
    }

    /// <summary>
    /// This class is needed as a workaround to http://support.microsoft.com/default.aspx?scid=kb;en-us;320369
    /// Excel automation will fail with the follwoing error on systems with non-English regional settings:
    /// "Old format or invalid type library. (Exception from HRESULT: 0x80028018 (TYPE_E_INVDATAREAD))" 
    /// </summary>
    class UILanguageHelper : IDisposable
    {
        private CultureInfo _currentCulture;

        public UILanguageHelper()
        {
            // save current culture and set culture to en-US 
            _currentCulture = System.Threading.Thread.CurrentThread.CurrentCulture;
            System.Threading.Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");
        }

        public void Dispose()
        {
            // reset to original culture 
            System.Threading.Thread.CurrentThread.CurrentCulture = _currentCulture;
        }
    }

    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 ExcelWindow 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() == "EXCEL7")
            {
                lParam = hwndChild;
                return false;
            }
            return true;
        }

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

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

                // Search the accessible child window (it has class name "EXCEL7") 
                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}");
                    ExcelWindow ptr;

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

                    if (hr >= 0)
                    {
                        // We successfully got a native OM IDispatch pointer, we can QI this for
                        // an Excel Application using reflection (and using UILanguageHelper to 
                        // fix http://support.microsoft.com/default.aspx?scid=kb;en-us;320369)
                        //
                        using (UILanguageHelper fix = new UILanguageHelper())
                        {
                            object xlApp = ptr.GetType().InvokeMember("Application", BindingFlags.GetProperty, null, ptr, null);

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

And this would be the same solution without PIAs in VB (Note that OM call are much more readable; however, the code to get access to the OM would be the same):

Option Strict Off

Imports System.Globalization
Imports System.Runtime.InteropServices
Imports System.Text

Module ExcelLateBindingSample

    ''' <summary>
    ''' Interface definition for Excel.Window interface
    ''' </summary>
    <Guid("00020893-0000-0000-C000-000000000046"), _
    InterfaceType(ComInterfaceType.InterfaceIsIDispatch)> _
    Public Interface ExcelWindow
    End Interface

    ''' <summary>
    ''' This class is needed as a workaround to http://support.microsoft.com/default.aspx?scid=kb;en-us;320369
    ''' Excel automation will fail with the follwoing error on systems with non-English regional settings:
    ''' "Old format or invalid type library. (Exception from HRESULT: 0x80028018 (TYPE_E_INVDATAREAD))" 
    ''' </summary>
    Class UILanguageHelper
        Implements IDisposable

        Private _currentCulture As CultureInfo

        Public Sub New()
            ' save current culture and set culture to en-US 
            _currentCulture = System.Threading.Thread.CurrentThread.CurrentCulture
            System.Threading.Thread.CurrentThread.CurrentCulture = New CultureInfo("en-US")
        End Sub

        Public Sub Dispose() Implements System.IDisposable.Dispose
            'reset to original culture 
            System.Threading.Thread.CurrentThread.CurrentCulture = _currentCulture
        End Sub

    End Class

    <DllImport("user32.dll", SetLastError:=True, CharSet:=CharSet.Auto)> _
    Private Function FindWindow(ByVal lpClassName As String, ByVal lpWindowName As String) As IntPtr
    End Function

    <DllImport("Oleacc.dll")> _
    Private Function AccessibleObjectFromWindow(ByVal hwnd As Integer, ByVal dwObjectID As UInt32, ByVal riid() As Byte, ByRef ptr As ExcelWindow) As Integer
    End Function

    Public Delegate Function EnumChildCallback(ByVal hwnd As Integer, ByRef lParam As Integer) As Boolean

    <DllImport("User32.dll")> _
    Public Function EnumChildWindows(ByVal hWndParent As Integer, ByVal lpEnumFunc As EnumChildCallback, ByRef lParam As Integer) As Boolean
    End Function

    <DllImport("User32.dll")> _
    Public Function GetClassName(ByVal hWnd As Integer, ByVal lpClassName As StringBuilder, ByVal nMaxCount As Integer) As Integer
    End Function

    Public Function EnumChildProc(ByVal hwndChild As Integer, ByRef lParam As Integer) As Boolean
        Dim buf As New StringBuilder(128)
        GetClassName(hwndChild, buf, 128)
        If buf.ToString() = "EXCEL7" Then
            lParam = hwndChild
            Return False
        End If
        Return True
    End Function

    Sub Main()
        ' Use the window class name ("XLMAIN") to retrieve a handle to Excel's main window.
        ' Alternatively you can get the window handle via the process id:
        ' Dim hwnd As Integer = CInt(Process.GetProcessById(excelPid).MainWindowHandle);
        '
        Dim hwnd As Integer = CInt(FindWindow("XLMAIN", Nothing))

        If hwnd <> 0 Then
            Dim hwndChild As Integer = 0

            ' Search the accessible child window (it has class name "EXCEL7") 
            Dim cb As New EnumChildCallback(AddressOf EnumChildProc)
            EnumChildWindows(hwnd, cb, hwndChild)

            If hwndChild <> 0 Then
                ' 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 OBJID_NATIVEOM As UInteger = &HFFFFFFF0&
                Dim IID_IDispatch As New Guid("{00020400-0000-0000-C000-000000000046}")
                Dim ptr As ExcelWindow

                Dim hr As Integer = AccessibleObjectFromWindow(hwndChild, OBJID_NATIVEOM, IID_IDispatch.ToByteArray(), ptr)

                If hr >= 0 Then
                    ' We successfully got a native OM IDispatch pointer, we can QI this for
                    ' an Excel Application using reflection (and using UILanguageHelper to 
                    ' fix http://support.microsoft.com/default.aspx?scid=kb;en-us;320369)
                    '
                    Using fixCrash As New UILanguageHelper
                        Console.WriteLine(String.Format("Excel version is: {0}", ptr.Application.Version))
                    End Using
                End If
            End If
        End If

    End Sub

End Module