How to Resolve 'Access Denied' Errors When Embedding an Unmanaged DLL in a Managed C# DLL?

Embedding an Unmanaged DLL within a Managed C# DLL

Embedding an unmanaged DLL within a managed C# DLL is a convenient way to bundle necessary resources and reduce dependency management. This question addresses an "Access Denied" exception encountered while attempting to embed an unmanaged DLL using the DllImport attribute.

The provided solution involves extracting the unmanaged DLL into a temporary directory during initialization. This step involves accessing the embedded resource stream and copying the DLL's contents to the temporary location. The extracted DLL is then explicitly loaded using LoadLibrary before using P/Invoke calls.

// Extract and load the unmanaged DLL.
string dllPath = Path.Combine(Path.GetTempPath(), "MyAssembly." +
  Assembly.GetExecutingAssembly().GetName().Version.ToString(), "MyAssembly.Unmanaged.dll");

using (Stream stm = Assembly.GetExecutingAssembly().GetManifestResourceStream(
  "MyAssembly.Properties.MyAssembly.Unmanaged.dll"))
{
  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);
    }
  }
}

IntPtr h = LoadLibrary(dllPath);
Debug.Assert(h != IntPtr.Zero, "Unable to load library " + dllPath);

Once the unmanaged DLL is loaded, it is available for use by DllImport calls within the managed DLL. This approach ensures that the correct version of the unmanaged DLL is used, even if multiple applications with different versions are running simultaneously.

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