How to Embed Unmanaged DLLs in Managed C# DLLs and Avoid 'Access is Denied'?

Embedding Unmanaged DLLs in Managed C# DLLs

When integrating unmanaged DLLs with managed C# code, developers often encounter the need to embed these DLLs within the managed assembly. This article investigates the potential issues and provides a solution for embedding unmanaged DLLs into managed DLLs.

Problem Statement

A developer attempts to embed an unmanaged DLL within a managed C# DLL using the DllImport attribute, as recommended by Microsoft. However, upon running the code, an "Access is denied" exception is thrown.

Solution

While the MSDN documentation suggests the feasibility of embedding unmanaged DLLs, it fails to address the underlying issue related to DLL access permissions. The following solution effectively addresses this problem:

1. Extract the Unmanaged DLL: During initialization, extract the unmanaged DLL from the embedded resource to a temporary directory.
2. Load the DLL Explicitly: After extraction, use the LoadLibrary function to explicitly load the DLL into the process.
3. Utilize DllImport: Once the DLL is loaded, DllImport directives referencing it will utilize the already loaded version.

Here's an example implementation of this approach:

// Extract the unmanaged DLL
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");
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);
    }
  }
}

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

By following these steps, developers can successfully embed unmanaged DLLs into managed C# DLLs, overcoming the "Access is denied" exception and unlocking the full potential of this integration technique.

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