PROMS/EPocalipse.IFilter/ComHelper.cs

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+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);
+    }
+  }
+}