How to Resolve 'Access Denied' Errors When Embedding Unmanaged DLLs in Managed C# DLLs?

How to Embed Unmanaged DLLs in Managed C# DLLs

Background

Embedding unmanaged DLLs within managed C# DLLs is a technique that allows for the seamless use of native code within .NET projects. Microsoft supports this feature, as outlined here:

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

Error: Access Denied (0x80070005)

Despite following the recommended approach, users may encounter the following exception:

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

Solution: Extract and Load DLL Explicitly

To resolve this issue, the unmanaged DLL should be extracted to a temporary directory and loaded explicitly using LoadLibrary before using P/Invoke. This technique ensures that resources are properly accessed and the dependency is established correctly.

Code Sample

The following code demonstrates this approach:

// Create a temporary directory
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");

// Extract the embedded 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 this approach, developers can embed unmanaged DLLs within managed C# DLLs and successfully utilize P/Invoke calls without encountering access denied errors.

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