Conquering the Exception on Loading C# DLL on a Secondary Thread in C++ MFC: A Step-by-Step Guide

Have you ever encountered the frustrating exception “LoaderLock was detected” when attempting to load a C# DLL on a secondary thread in a C++ MFC application? Fear not, dear developer, for you are not alone! This article will walk you through the treacherous landscape of interop, threading, and DLL loading, providing a comprehensive solution to this pesky problem.

The Problem: A Brief Overview

When a C# DLL is loaded on a secondary thread in a C++ MFC application, the .NET runtime may throw a LoaderLock exception. This occurs because the .NET runtime is not thread-safe by default, and the MFC framework is not designed to handle the complexities of .NET threading.

Why Does This Happen?

The LoaderLock exception arises from the way .NET loads assemblies and resolves dependencies. When a C# DLL is loaded on a secondary thread, the .NET runtime attempts to acquire a lock on the loader, which is already held by the main thread. This results in a deadlock, causing the exception to be thrown.

The Solution: A Step-by-Step Approach

To overcome this hurdle, we’ll employ a combination of threading techniques, DLL loading strategies, and clever use of .NET APIs. Follow along, and we’ll get your C# DLL loaded on a secondary thread in no time!

Step 1: Create a Custom Loader Class

In your C++ MFC project, create a new class responsible for loading the C# DLL. This class will serve as an intermediary between the MFC framework and the .NET runtime.

class CDLLLoader {
public:
    CDLLLoader();
    ~CDLLLoader();

    HRESULT LoadDLL(const CString& dllPath);
    void UnloadDLL();

private:
    // Member variables and implementations omitted for brevity
};

Step 2: Initialize the .NET Runtime

In the constructor of the `CDLLLoader` class, initialize the .NET runtime using the following code:

CDLLLoader::CDLLLoader() {
    // Initialize the .NET runtime
    CorBindToRuntimeHost(&m_pRuntimeHost);
    m_pRuntimeHost->Start();

    // Create a new AppDomain
    m_pAppDomain = m_pRuntimeHost->CreateDomain("MyAppDomain");
}

Step 3: Load the C# DLL

Implement the `LoadDLL` method to load the C# DLL using the following code:

HRESULT CDLLLoader::LoadDLL(const CString& dllPath) {
    // Load the DLL into the current AppDomain
    m_pAppDomain->Load_2(dllPath);

    // Get the type of the C# class we want to instantiate
    m_pType = m_pAppDomain->GetType("MyNamespace.MyClass");

    return S_OK;
}

Step 4: Create a Thread-Safe Instance of the C# Class

Create a thread-safe instance of the C# class using the following code:

CMyClass* CDLLLoader::CreateInstance() {
    // Create a new instance of the C# class
    m_pInstance = m_pAppDomain->CreateInstanceAndUnwrap(m_pType);

    return m_pInstance;
}

Step 5: Unload the C# DLL

Implement the `UnloadDLL` method to unload the C# DLL using the following code:

void CDLLLoader::UnloadDLL() {
    // Unload the AppDomain
    m_pAppDomain->Unload();

    // Release the .NET runtime
    m_pRuntimeHost->Stop();
}

Putting it all Together

Now that we have our custom loader class, let’s see how to use it to load the C# DLL on a secondary thread:

void MyThreadProc() {
    // Create a new instance of the custom loader class
    CDLLLoader loader;

    // Load the C# DLL
    loader.LoadDLL("MyCSharpDLL.dll");

    // Create a thread-safe instance of the C# class
    CMyClass* instance = loader.CreateInstance();

    // Use the instance...
    instance->DoSomething();

    // Unload the DLL
    loader.UnloadDLL();
}

Tips and Tricks

Here are some additional tips to keep in mind when working with C# DLLs and MFC:

• Make sure to set the correct threading model for your MFC application. In this case, we’ll use the “Single-threaded” model.

• When loading the C# DLL, ensure that the DLL is built with the correct architecture (x86 or x64) to match your MFC application.

• If you’re using a 64-bit version of Windows, make sure to install the correct version of the .NET Framework (x86 or x64) to match your application.

Conclusion

By following this step-by-step guide, you should now be able to load a C# DLL on a secondary thread in a C++ MFC application without encountering the LoaderLock exception. Remember to initialize the .NET runtime, load the DLL into an AppDomain, create a thread-safe instance of the C# class, and unload the DLL when you’re finished. With these techniques, you’ll be well on your way to harnessing the power of .NET in your MFC applications.

Keyword	Count
Exception on loading C# dll on a secondary thread in C++ MFC	5
C# DLL	7
C++ MFC	5
LoaderLock	2
.NET runtime	4

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Happy coding, and may the exceptions be ever in your favor!

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