How Can I Embed an Unmanaged DLL within a Managed C# DLL?

Embedding Unmanaged DLLs into Managed C# DLLs

Problem:

How can you embed an unmanaged C DLL within a managed C# DLL using DLLImport?

Answer:

Although embedding unmanaged DLLs into managed DLLs is generally advised against, it can be achieved by extracting the unmanaged DLL to a temporary directory during initialization and explicitly loading it with LoadLibrary before using P/Invoke.

Implementation:

1. Extract the Unmanaged DLL: Determine a temporary directory to store the unmanaged DLL, using the assembly's version number to avoid version conflicts. Extract the embedded resource stream containing the DLL to this directory.
2. Load the DLL: Explicitly load the extracted DLL using LoadLibrary. The temporary directory may not be in the system path, so this is necessary for P/Invoke directives to locate the DLL.

Example Code:

// Get temporary directory with assembly version in path
string dirName = Path.Combine(Path.GetTempPath(), "MyAssembly." + Assembly.GetExecutingAssembly().GetName().Version);
Directory.CreateDirectory(dirName);
string dllPath = Path.Combine(dirName, "MyAssembly.Unmanaged.dll");

// Get embedded resource stream and copy DLL to temporary file
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 DLL explicitly
IntPtr h = LoadLibrary(dllPath);
Debug.Assert(h != IntPtr.Zero, "Unable to load library " + dllPath);

Note that this approach has its advantages, such as keeping everything in a single file, but linking the unmanaged DLL to the assembly as a separate file is generally recommended for both security and simplicity.

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