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Call C++ code from C# without creating dll?

I'm noob in C# and I already searched on the web. Anyway I'm still not sure about this and I do not have the total control of the code I have to implement, this is why I would like to be sure I needn't make any dll. I have a C++ file with a method, and I want to call this method from C# code. I just added "extern C" to the function.
When I just add the .h and .cpp files to the C# project they aren't detected. And of course, when I try to add it as reference, it doesn't work.
So do I absolutly have to make a dll ?

Your options for accessing the C++ code from C#:
Compile C++ as unmanaged DLL and access using p/invoke. This requires the C++ code be exposed using a C style API.
Compile C++ as unmanaged DLL and access using COM. This requires that you wrap your C++ in as COM objects.
Compile C++ as mixed/mode C++/CLI assembly and access the assembly as a managed reference. This requires that you wrap the original C++ as managed C++ ref classes.
All of these options, by necessity, involve the creation of another module/assembly. You cannot link the C++ code directly into your C# assembly.

You might like to try using the PInvoke Interop Assistant to generate the C# necessary to interact with the DLL via Platform Invoke. Be aware that this is imperfect though so YMMV.
Another alternative, if you have the knowledge and patience, is to make a COM component out of your native C++ DLL and consume that from C# by using the Type Library Importer to create a managed wrapper.

You won't be able to interact with .cpp/.h files since you need at least a binary object (assembly) for C# to interact with and C# won't generate any binaries from .cpp/.h. That's on the subject about adding these files as references to the project.
As for the argument that you don't have control over the code - well, don't make a DLL out of the actual .cpp/.h, but make your own thin DLL that has a wrapper object that just includes the headers, calls whatever method you would be calling and links to the appropriate .o files or .lib or whatever you have. If the interface changes you would just changed your thing wrapper which should be easy.

how to Pass C++ Object to C# Class Library

I am working on a project. In that C++ code is referencing to a Class Library which is written in C#. I want to pass Object of C++ Classes to C# Class library.
So Is this possible, If yes please let me know how ?? otherwise I will have to pass around 100 arguments to C# Class Library.
Regards,
Vivek

you shoul use marshling.
Marshaling is the process of creating a bridge between managed code and unmanaged code; it is the homer that carries messages from the managed to the unmanaged environment and reverse. It is one of the core services offered by the CLR (Common Language Runtime.)
namespace System.Runtime.InteropServices.Marshal
Read here a good blog about marshling

If you have this option, you can add a cpp file to your project that will be compiled with enabled CLR (managed C++). From managed C++ you can call C# classes. You can find and example here:
Managed C++ to form a bridge between c# and C++
Another option is to create a managed C++ dll as a bridge between C++ and .NET if you don't want to enable CLR in your calling native application. I used this trick and it worked fine but be aware that once you load that bridge dll library, CLR is loaded in memory and your process gets "infected" by CLR. You will have that effect anyway though.

Creating a DLL that can be used by native and managed applications

Even after searching a while I could not find a clear answer to this. It seems to be somewhat of an uncommon problem:
I have a native implementation that should go in a DLL. That DLL should be usable by both native applications as well as managed applications.
So if I write this DLL in C++/CLI I could expose two things:
A flat, C-style exported interface of functions that could be used by native applications.
A Managed class that can be used from any managed application (e.g. C#) by referencing this DLL
Will this work? Will purely native applications be able to load this DLL and call the exposed functions?

The native DLL is compiled like a regular native DLL (no /clr).
The calling C++/CLI application/DLL uses a C++ class to load the DLL via LoadLibrary and GetProcAdress.
You need to have the above C++ class map all the functions of the native DLL.
The C++ class acts as a proxy class:
Example:
Native DLL hasa function foo().
C++/CLI code exposes a method foo which looks like:
void MyProxy::foo() {
m_foo(); // m_foo is a function pointer to foo() in the DLL obtained via GetProcAddress.
}

How to interop with C++ dlls when I already have Interface and Type definitions

I would like to use a set of unmanaged C++ libraries for a large project in my C# .net application. After downloading the code for the libraries, I noticed that the demo project that uses the libraries was actually done in C++/CLI.
Since the demo project was done in C++/CLI the author wen't ahead and added most of the type definitions and interface definitions in header files in the external dependencies folder of this demo application.
If I can, how exactly can I use these definitions to interop with these managed C++ dlls?
To recap:
Libraries are written in unmanaged C++
Demo project is written in managed C++ has type definitions and interface definitions created to use the unamanged C++ dlls, they are located in the "External Dependencies" folder and are header files(.h).
I want to use these unmanaged C++ dlls in my C# project, can I somehow use the definitions to interop with them?
4.If not, what would be my best option to interop with them? COM interop?

I think you are looking for this....
This is an example from the FMod DLL wrapper for c#
[DllImport (VERSION.dll)]
private static extern RESULT FMOD_DSPConnection_GetInput (IntPtr dspconnection, ref IntPtr input);
I know it is a lot of work but way better than rewriting the unmanaged DLL.
Not knowing which library you intend to use I can only hope this helps

How can I add a VC++ DLL as a reference in my C# Visual Studio project?

I want to add a VC++ DLL reference into my C# Visual Studio project. But when I try to add it I see, "It is not a valid assembly or COM component".
Please suggest how I can use the VC++ DLL as a reference in a C# project.

There are two options for using a C++ DLL from C#: either COM interop, or P/Invoke. COM Interop involves creating a COM object in your C++ DLL, and then adding it as a reference. You can use the COM object like a C# object (for the most part) at this point.
P/Invoke allows you to call exported functions from C# (think calling standard Win32 API functions from C#). This is likely easier to set up, since all you need to do is export a function, however that could cause you to refactor your code in the C++ DLL, since it's not a very OOP way of doing things.

You can only use C++ components in C# when they have been prepared for use, for example by being written in C++/CLI or being written as a COM server.
If your component is a plain C++ dll you'll need to write some wrapper code, probably best is C++/Cli

I am not sure whether this solve..
run:
tlbimp /out:MyOldCom.dll MyNewAssembly.tlb
Then use it as you would any other assembly.
Please refer
http://msdn.microsoft.com/en-us/library/aa302324.aspx
http://msdn.microsoft.com/en-us/magazine/cc301501.aspx
ie)
One way is to package your DLL as a COM class and Another way is using DllImport