Embedding unmanaged dll into a managed C# dll

Question

I have a managed C# dll that uses an unmanaged C++ dll using DLLImport. All is working great. However, I want to embed that unmanaged DLL inside my managed DLL as explain by Microsoft there:

http://msdn.microsoft.com/en-us/library/system.runtime.interopservices.dllimportattribute.dllimportattribute.aspx

So I added the unmanaged dll file to my managed dll project, set the property to 'Embedded Resource' and modify the DLLImport to something like

[DllImport("Unmanaged Driver.dll, Wrapper Engine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", CallingConvention = CallingConvention.Winapi)]

where 'Wrapper Engine' is the assembly name of my managed DLL 'Unmanaged Driver.dll' is the unmanaged DLL

When I run, I get: Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))

I saw from MSDN and from http://blogs.msdn.com/suzcook/ that's supposed to be possible...

Any idea? Thanks

Answer 1

I wasn't aware this is possible - I'd guess that the CLR needs to extract the embedded native DLL somewhere (Windows needs to have a file for the DLL to load it - it cannot load an image from raw memory), and wherever it's trying to do that the process does not have permission.

Something like Process Monitor from SysInternals might give you a clue if the pronblem is that creating the DLL file is failing...

Update:

---

Ah... now that I've been able to read Suzanne Cook's article (the page didn't come up for me before), note that she is not talking about embedding the native DLL as a resource inside the managed DLL, but rather as a linked resource - the native DLL still needs to be its own file in the file system.

See http://msdn.microsoft.com/en-us/library/xawyf94k.aspx, where it says:

The resource file is not added to the output file. This differs from the /resource option which does embed a resource file in the output file.

What this seems to do is add metadata to the assembly that causes the native DLL to logically be part of the assembly (even though it's physically a separate file). So things like putting the managed assembly into the GAC will automatically include the native DLL, etc.

Answer 2

You can embed the unmanaged DLL as a resource if you extract it yourself to a temporary directory during initialization, and load it explicitly with LoadLibrary before using P/Invoke. I have used this technique and it works well. You may prefer to just link it to the assembly as a separate file as Michael noted, but having it all in one file has its advantages. Here's the approach I used:

// Get a temporary directory in which we can store the unmanaged DLL, with
// this assembly's version number in the path in order to avoid version
// conflicts in case two applications are running at once with different versions
string dirName = Path.Combine(Path.GetTempPath(), "MyAssembly." +
  Assembly.GetExecutingAssembly().GetName().Version.ToString());
if (!Directory.Exists(dirName))
  Directory.CreateDirectory(dirName);
string dllPath = Path.Combine(dirName, "MyAssembly.Unmanaged.dll");

// Get the embedded resource stream that holds the Internal DLL in this assembly.
// The name looks funny because it must be the default namespace of this project
// (MyAssembly.) plus the name of the Properties subdirectory where the
// embedded resource resides (Properties.) plus the name of the file.
using (Stream stm = Assembly.GetExecutingAssembly().GetManifestResourceStream(
  "MyAssembly.Properties.MyAssembly.Unmanaged.dll"))
{
  // Copy the assembly to the temporary file
  try
  {
    using (Stream outFile = File.Create(dllPath))
    {
      const int sz = 4096;
      byte[] buf = new byte[sz];
      while (true)
      {
        int nRead = stm.Read(buf, 0, sz);
        if (nRead < 1)
          break;
        outFile.Write(buf, 0, nRead);
      }
    }
  }
  catch
  {
    // This may happen if another process has already created and loaded the file.
    // Since the directory includes the version number of this assembly we can
    // assume that it's the same bits, so we just ignore the excecption here and
    // load the DLL.
  }
}

// We must explicitly load the DLL here because the temporary directory 
// is not in the PATH.
// Once it is loaded, the DllImport directives that use the DLL will use
// the one that is already loaded into the process.
IntPtr h = LoadLibrary(dllPath);
Debug.Assert(h != IntPtr.Zero, "Unable to load library " + dllPath);

Answer 3

Access is denied. (Exception from HRESULT: 0x80070005 (E_ACCESSDENIED))

You will get this error message if your unmanaged embedded DLL calls another DLL and cannot be found. Have you explored this possibility?

Answer 4

http://stackoverflow.com/questions/72264/how-can-a-c-windows-dll-be-merged-into-a-c-application-exe