Develop Reference

Marshaling a Struct with methods using C# P/Invoke

I'm trying to PInvoke into this C++ library function:
int Initialize(Callback* callback);
struct Callback
{
//.......
void virtual StateChanged(int state) = 0;
};
I have tried this naive C# code, but it doesn't work.
[DllImport("CPlusPlusLibrary.dll", SetLastError = true, EntryPoint = "?Initialize#CPlusPlusLibrary##YAHPAUCallback#1##Z")]
public static extern int Initialize(Callback callback);
[StructLayout(LayoutKind.Sequential)]
public class Callback
{
//....
public delegate void IntDelegate(int state);
public IntDelegate StateChanged(int state);
}
var callback = new Callback();
var result = Initialize(callback);

It is impossible to do it that way as far as I know. Methods are not "in there" as fields would be, beside this, creating a struct with virtual method will create a vtable pointer in your objects, that you are not taking into account in c# mirrored class.
What you can do is to PInvoke to method that takes a functionPointer (in C++) and pass a delegate (C#) there. You can then use this function pointer to call it from native code and your delegate will launch.
You could then change your stateChange method definition to take a Callback* as a first parameter, so when you call it from native code you can pass an object pointer which is responsible of that change and marshal it back to Callback in c#.
//Edit without having source of native dll, building a bridge between c# and c++ is what comes to my mind. This can be done with c++/cli or c++, sticking to native my idea would be something like this:
//c++ <--> new c++ dll <--> c#
struct CallbackBridge : public Callback
{
void (*_stateChanged)(int);
virtual void stateChanged(int state)
{
if (_stateChanged)
_stateChanged(this, state);
}
};
void* CreateCallback() { return new CallbackBridge(); }
void DeleteCallback(void* callback); { delete callback; }
void setStateChanged(void* callback, void (*ptr)(void*, int))
{
CallbackBridge* bridge = (CallbackBridge*)callback;
bridge->stateChanged = ptr;
}
///... other helper methods
The idea here is to treat your object as a black box (hence void* everywhere - it can be any pointer, but from c# you will just see this a a SafeHandle / IntPtr and write helper methods that you can PInvoke to to create / delete and modify objects. You can mock those virtual calls by giving your object a delegate through such method.
From c# usage could look like this: (IntPtr for simplicity, SafeHandle could be used):
IntPtr callback = CreateCallback();
SetStateChanged(callback, myCallback);
//somewhere later:
DeleteCallback(callback);
void MyCallback(IntrPtr callback, int state)
{
int someData = SomeOtherHelperMethod(callback);
ConsoleWrite("SomeData of callback is {0} and it has changed it's state to {1}", someData, state);
}
I know, it's a bit clumsy for bigger objects, but without c++/cli wrapper I have never found any better way to be able to handle all those tricky cases like virtual calls etc.

Your C# class is no substitute for the C++ struct, it has an entirely different internal organization. The C++ struct has a v-table because of the virtual method. It is not a POD type anymore. It has a hidden field that contains a pointer to the v-table. And a compiler-generated constructor and destructor. The v-table is an array of pointers to the virtual methods. A C# class has something similar, called "method table", but it organized completely differently. It will have the System.Object methods for example and contains pointers to all methods, not just the virtual ones. The class object layout is completely different as well. The struct keyword in the C++ language in this context is just a substitute for the class keyword with all members public by default.
A wrapper class written in the C++/CLI language is required. There's a bit of a learning curve to the language but it is not steep if you've got experience with .NET and C++. Boilerplate code for such a wrapper is in this answer. You can call back from native code into managed code through a function pointer returned by Marshal::GetFunctionPointerForDelegate().

I have heard that you don't have the source code of C++ library. Then you might have a look at this.
But exporting classes from unmanaged dll to managed one without source sounds dangerous to me.

How to use C++ class instance in C#

I have a DLL that contains a class that inherits from another abstract C++ class defined as following:
class PersonInterface
{
public:
virtual int __stdcall GetName() = 0;
};
The DLL exports a function that can be used in C# as following (following method is part of static class PersonManager):
[DllImport( "person.dll", CallingConvention = CallingConvention.Cdecl )]
public static extern bool GetPerson( out PersonInterface person );
where PersonInterface is defined in C# class as following:
[StructLayout( LayoutKind.Sequential )]
public class PersonInterface
{
}
I can successfully retrieve the C++ class instance like this:
PersonInterface person;
bool retrieved = PersonManager.GetPerson( out person );
However, the retrieved object is not of any use until GetName method can be called.
What else needs to be done in order to be able to be able to invoke GetName method on retrieved person object?
string name = person.GetName();

Compile your C++ DLL using /clr compiler option.
Write up a managed class using public ref class syntax, and have pointer to your native class in this class. Expose all methods from this managed class and forward all calls to your native class.
Import this DLL as assembly in your c# project and use this class as you would use any other .NET class.
You need to compile only few source files using /clr flag, not all. Let all native source files be compiled as native. Your DLL will be linked to VC runtime DLL as well as .NET runtime DLL.
There is lot to managed class, /clr, Interoperability/Marshalling, but at least get started.
You may choose your favorite articles from here

You`ll have to write a wrapper in C++ (it may be managed C++), where you call you C++ clasess and expose either flat dll functions, which can be called from .Net, or .Net classes (if you used managed C++), that will be accessible from .Net.

Interop with another function in a different class

I would have to call C++ code from .Net code via interop.
I just wonder whether is there anyway to interop with another function in a different class? For example, in C++, I have the following utility class:
class ConvertUtility
{
public:
static void Convert(PointList &ptList, const list<pts> &pts);
};
I wish to call it directly from .Net via interop, any idea how to do this?
Note: here's a related question asking about how to use namespace to distinguish between different method. But this time, I want nothing to do with namespace, only a class with static function.
Edit: Given that there are already too many functions in the C wrapper ( e.g, static extern "C" function that are callable from .Net, without class or namespace), I won't want to introduce an extra layer of wrapping, if I can help it.

In the related question you linked to, Ben Voigt says in a comment to the suggestion to write a C++/CLI wrapper:
This IS the correct answer. P/Invoke should only be used to call
functions with a "C" interface, which means extern "C" to prevent name
mangling, and also restrictions on parameter and return types.
Since the method is static, I see two options:
Write a simple C wrapper function that can be called with P/Invoke.
Write a C++/CLI wrapper that can be called directly from C#.

Calling c++ assemblies with C#

I have a c++ dll which I need to use in my .NET code.
How do I call methods on this library, and use the result in my code?
e.g. C++ library has a method called Move(int) which returns a 1 for true and a 0 for false.
I didn't write the C++ library, it is used to control the movement of a robotic machine.

You could use P/Invoke. So for example if your unmanaged library exports a function which takes an integer value as argument and returns another integer you could define a managed wrapper for this method in C# decorating it with the [DllImport] attribute:
[DllImport("foo.dll")]
public static extern int Move(int arg);
and then use this method directly in managed code:
int result = Move(123);

http://msdn.microsoft.com/en-us/library/ms173184.aspx
using System.Runtime.InteropServices;
...
public class MyClass
{
#region PInvokes
[DllImport("Your.dll")]
static public extern int Move(int val);
....
If, once, you would have to Interop C++ classes, here is a good article about it.
http://www.codeproject.com/KB/cs/marshalCPPclass.aspx

Using a C++ class in C#

How can I use a C++ class in C#? I know you can use DllImport and have the class as IntPtr, but then how would I call the functions inside the class? And I don't want to create a new method for each method in the class and export that, then there is not point for me to use a class then...
Without creating a COM app
C++ Class:
class MyCPPClass
{
public:
MyCPPClass();
~MyCPPClass();
void call_me();
}
extern "C"
{
extern __declspec(dllexport) MyCPPClass* __cdecl CreateClass()
{
return new MyCPPClass();
}
}
C#:
class MyCSClass
{
[DllImport("MyCPPDll.dll")]
static extern IntPtr CreateClass();
public static void Main()
{
IntPtr cc = CreateClass();
// Now I want to call cc->call_me();
}
}

If you're building your classes in C++/CLI, then you just just be able to use it as you would any regular .NET class.
If the DLL in question is accessible via COM, then use the COM interop to talk to it from .NET.
If OTOH you're looking at non-COM, plain vanilla C++, you will have to ensure that all classes and their members are exported...
My suggestion would be that if you want to use a plain vanilla C++ DLL, write a C++/CLI wrapper around it that can "talk .NET" to your C# code and "real C++" to the C++ DLL. You'll still have to export all the C++ classes and functions, but at least that way you're working in the same language before you transition to C#.

To use c++ class, C++\CLI is a better option

As others have stated, C++ CLI is probably the easiest option.
The other standard option which you've started doing is to keep building on that C style facade you started and expose another method something like this:
extern "C" {
extern __declspec(dllexport) MyCPPClass* __cdecl CallMe(void* data)
{
MyCppClass* instance = (MyCppClass*) data;
instance->call_me();
}
}
then you can go through the tedium of creating a C# class that mirrors the functionality of your C++ class and hides the fact that the implementation is in another dll.
Or you could use C++ / CLI.