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Using IFilter in C#

, 19 Mar 2006 Ms-PL
Using the IFilter interface to extract text from various document types.
ifilter.zip
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EPocalipse.IFilter
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Release
EPocalipse.IFilter.dll
Properties
Tester
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Release
EPocalipse.IFilter.dll
IFilterTester.exe
Properties
using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace EPocalipse.IFilter
{
  [ComVisible(false)]
  [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown), Guid("00000001-0000-0000-C000-000000000046")]
  internal interface IClassFactory
  {
    void CreateInstance([MarshalAs(UnmanagedType.Interface)] object pUnkOuter, ref Guid refiid, [MarshalAs(UnmanagedType.Interface)] out object ppunk);
    void LockServer(bool fLock);
  }

  /// <summary>
  /// Utility class to get a Class Factory for a certain Class ID 
  /// by loading the dll that implements that class
  /// </summary>
  internal static class ComHelper
  {
    //DllGetClassObject fuction pointer signature
    private delegate int DllGetClassObject(ref Guid ClassId, ref Guid InterfaceId, [Out, MarshalAs(UnmanagedType.Interface)] out object ppunk);

    //Some win32 methods to load\unload dlls and get a function pointer
    private class Win32NativeMethods
    {
      [DllImport("kernel32.dll", CharSet=CharSet.Ansi)]
      public static extern IntPtr GetProcAddress(IntPtr hModule, string lpProcName);

      [DllImport("kernel32.dll")]
      public static extern bool FreeLibrary(IntPtr hModule);

      [DllImport("kernel32.dll")]
      public static extern IntPtr LoadLibrary(string lpFileName);
    }

    /// <summary>
    /// Holds a list of dll handles and unloads the dlls 
    /// in the destructor
    /// </summary>
    private class DllList
    {
      private List<IntPtr> _dllList=new List<IntPtr>();
      public void AddDllHandle(IntPtr dllHandle)
      {
        lock (_dllList)
        {
          _dllList.Add(dllHandle);
        }
      }

      ~DllList()
      {
        foreach (IntPtr dllHandle in _dllList)
        {
          try
          {
            Win32NativeMethods.FreeLibrary(dllHandle);
          }
          catch { };
        }
      }
    }

    static DllList _dllList=new DllList();

    /// <summary>
    /// Gets a class factory for a specific COM Class ID. 
    /// </summary>
    /// <param name="dllName">The dll where the COM class is implemented</param>
    /// <param name="filterPersistClass">The requested Class ID</param>
    /// <returns>IClassFactory instance used to create instances of that class</returns>
    internal static IClassFactory GetClassFactory(string dllName, string filterPersistClass)
    {
      //Load the class factory from the dll
      IClassFactory classFactory=GetClassFactoryFromDll(dllName, filterPersistClass);
      return classFactory;
    }

    private static IClassFactory GetClassFactoryFromDll(string dllName, string filterPersistClass)
    {
      //Load the dll
      IntPtr dllHandle=Win32NativeMethods.LoadLibrary(dllName);
      if (dllHandle==IntPtr.Zero)
        return null;

      //Keep a reference to the dll until the process\AppDomain dies
      _dllList.AddDllHandle(dllHandle);

      //Get a pointer to the DllGetClassObject function
      IntPtr dllGetClassObjectPtr=Win32NativeMethods.GetProcAddress(dllHandle, "DllGetClassObject");
      if (dllGetClassObjectPtr==IntPtr.Zero)
        return null;

      //Convert the function pointer to a .net delegate
      DllGetClassObject dllGetClassObject=(DllGetClassObject)Marshal.GetDelegateForFunctionPointer(dllGetClassObjectPtr, typeof(DllGetClassObject));

      //Call the DllGetClassObject to retreive a class factory for out Filter class
      Guid filterPersistGUID=new Guid(filterPersistClass);
      Guid IClassFactoryGUID=new Guid("00000001-0000-0000-C000-000000000046"); //IClassFactory class id
      Object unk;
      if (dllGetClassObject(ref filterPersistGUID, ref IClassFactoryGUID, out unk)!=0)
        return null;

      //Yippie! cast the returned object to IClassFactory
      return (unk as IClassFactory);
    }
  }
}

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License

This article, along with any associated source code and files, is licensed under The Microsoft Public License (Ms-PL)

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About the Author

Eyal Post

Israel Israel
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