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I want to pass a .net class to a native function and work with that class (i.e. get the value of a Property). Note that i don't want to use C++/CLI.

For example, i have a Label in my web application and i want to get Text property in my native C++ code.

My Attempt

I tried to load CLR in C++ and read Text property of a label which is instantiated in C#, but i encountered System.AccessViolationException exception.

Here is my code:

C# 

static void Main(string[] args) { var label = new Label { Text = "Some Text" }; //Send Type because CLR Invocation is defined on object's Type var labelType = label.GetType(); GCHandle gch = GCHandle.Alloc(labelType); IntPtr labelTypeIntPtr = GCHandle.ToIntPtr(gch); ReadDotNetClass(labelTypeIntPtr); } //native function definition [DllImport("Unmanaged.dll")] private static extern void ReadDotNetClass(IntPtr labelTypeIntPtr);

C++ 

extern "C" __declspec(dllexport) void ReadDotNetClass(_TypePtr labelTypePtr) { PCWSTR pszVersion = L"v4.0.30319"; PCWSTR pszAssemblyName= L"System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"; PCWSTR pszClassName=L"System.Web.UI.WebControls.Label"; ICLRMetaHost *pMetaHost = NULL; ICLRRuntimeInfo *pRuntimeInfo = NULL; ICorRuntimeHost *pCorRuntimeHost = NULL; IUnknownPtr spAppDomainThunk = NULL; _AppDomainPtr spDefaultAppDomain = NULL; // The .NET assembly to load. bstr_t bstrAssemblyName(pszAssemblyName); _AssemblyPtr spAssembly = NULL; // The .NET class to instantiate. bstr_t bstrClassName(pszClassName); variant_t vtObject; // The instance method in the .NET class to invoke. bstr_t bstrMethodName(L"Text"); SAFEARRAY *psaMethodArgs = NULL; variant_t vtStringRet; CLRCreateInstance(CLSID_CLRMetaHost, IID_PPV_ARGS(&pMetaHost)); pMetaHost->GetRuntime(pszVersion, IID_PPV_ARGS(&pRuntimeInfo)); BOOL fLoadable; pRuntimeInfo->IsLoadable(&fLoadable); pRuntimeInfo->GetInterface(CLSID_CorRuntimeHost, IID_PPV_ARGS(&pCorRuntimeHost)); pCorRuntimeHost->Start(); pCorRuntimeHost->GetDefaultDomain(&spAppDomainThunk); spAppDomainThunk->QueryInterface(IID_PPV_ARGS(&spDefaultAppDomain)); spDefaultAppDomain->Load_2(bstrAssemblyName, &spAssembly); psaMethodArgs = SafeArrayCreateVector(VT_VARIANT, 0, 0); //Invoke method from the Type interface. //System.AccessViolationException occurred here HRESULT hr = labelTypePtr->InvokeMember_3(bstrMethodName, static_cast<BindingFlags>( BindingFlags_Instance | BindingFlags_Public | BindingFlags_GetProperty), NULL, vtObject, psaMethodArgs, &vtStringRet); if (FAILED(hr)) wprintf(L"Failed to invoke Method w/hr 0x%08lx\n", hr); }

The C++ code is simplified version of this code sample from microsoft.

Please note that i just need to deal with .net classes (not my own classes) in native C++, and i don't want to use COM or other methods that take place out side of native world.

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  • I know nothing about this, but as a starting point in locating the problem, exactly which statement in your C++ program gets the System.AccessViolationException? And is that a .Net exception or a C++ runtime exception? Commented Jul 26, 2014 at 12:55
  • I really don't understand what you're trying to do. In your C# program, is the Label object a WinForms Label? If so, it looks like you're trying to work with WinForms data without setting up the whole WinForms environment via Application.Run which starts the Windows message pump. And I really can't imagine that it is possible to load a .Net WinForms assembly into a native C++ program. It makes no sense. Commented Jul 26, 2014 at 13:03
  • What I said and my code was just an example. It is important for me to put a few line of code into C++ to prevent easily disassembling my C# application and changing important parts of my code. I commented where i got that exception, but i don't know what kind of exception that is. Also i noted that my example is about a Label in a Web application. Commented Jul 26, 2014 at 13:41
  • OK, now I'm really confused. (More than I usually am :-) If your objective is to prevent your .Net code from being disassembled, then this is not the way to do it. What you need is an obfuscation utility. There are many available, some are free and some are very expensive. Commented Jul 26, 2014 at 13:52
  • I spent a few month on obfuscation. But that does not help anyway. I see many .net assemblies were cracked, even by novice users. So, please trust me and focus on problem itself. :-) Commented Jul 26, 2014 at 14:00

1 Answer 1

Reset to default
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The .Label property is an instance property, yet you're not passing the created instance to the ReadDotNetClass function - just the handle to the Label's Type. Note that if you're calling the native function from managed code you don't need to initialize the CLR - it's already initialized, running your managed code. So, include a pointer to the managed object in the native function signature, pass it when calling the native function along with the pointer to the managed object's Type, and inside the native function just call InvokeMember on the Type instance, passing the managed object as the target parameter - remove all the CLR initialization stuff.

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4 Comments

I removed whole clr initializations code and I just pass object instance and it's type to VARIANT parameter type in C++ and it works fine. Thanks! But as you said, I should and I want to pass instance and it's type by pointer. but I don't know how to use pointer as target in InvokeMember.
Running the code sample you linked, I can see that the variant type needs to be VT_UNKNOWN. To get IUnknown pointer from a managed object instance, use Marshal.GetIUnknownForObject - don't use GCHandle.
I pass the pointer that i get by Marshal.GetIUnknownForObject to IUnknown* parameter, but I still don't know how to convert IUnknown* to VARIANT.
The _variant_t helper class has both overloaded assignment operator and a constructor that accepts IUnknown *. You should be able to write vtObject = pUnknown.

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