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What is the best way to port a C++ library to C#?

I have a (header only) C++ library that I am looking to port to C#. It is a wrapper of the win api which I have made for a certain purpose. I want to port it to C# because I want to develop it further using C#. What is the most efficient (and easiest?) way to port it. Please note that I do not want something hectic because I don't want to spend more time porting the library than it took to make that library in the first place. So is there a way?

It depends very much on how big your lib is, how it is structured, how your memory allocation looks like, how much you made use of libs not available in C# (like the C++ std lib), how much C++ template programming you have used etc.
Different strategies may be
try to port (parts of) it automatically by some code generator of your own
do not port it to C#, instead use C++/CLI (something I did very successfully in the past)
do it manually because the C++ concepts you have used so far don't map well to C#
"Header only" does not seem to make a real difference. In fact, things may technically get a little bit easier when you have just one C++ file for each class to be ported to one C# class file.

A couple of ways I can think of.
Manual conversion to C# dll with possible code generation help of T4.
Change your c++ library to a managed dll so you can use it in your c# project.
Of course, you might use interop with your c++ library in your C# project. But in that case, I am not sure about purpose of your c++ library since you said it's a wrapper.
Since you already have c++ library that you can fully control, I would try to go with #2 first.

what are the possible ways to using c# code in c++

I'm just looking for a best way to re-use code written in c#, in my c++ projects. Creating a com\service doesn't look like a best option for my needs. How difficult it is to export c# code into a dll and use it in c++? can i get some suggestion or example? is this usual requirement or ? Please help me.
i use win7, VS2008, win7sdk
Thanks & Rgds, ~calvin

Executing managed code from an unnamaged executable is possible, though not quite easy. You can look into this article for an introduction and this book to go further.
I personally would avoid this kind of things in most cases and, if possible, switch the C++ project to C++/CLI to obtain an immediate compatibility with .Net assemblies for a minimal cost.

You can't export C# code into a native dll afaik. At least without very much pain in your buttocks. You should have thought beforehand and write the reusable part in C, thus creating a native DLL which could be used from all languages.
Also, you could try managed C++ - I personally hate it... but there you go

In case you need to use code written in C# C++, then you need to first see what all data types you would be passing from your C++ code to C# code.
1. Basic data types like int, enum etc can be passed from unmanaged to managed code.
2. in case you want to pass on class object, than you need to use marshalling.

If you can't use COM (if the .NET code is already written for example), then you can host the CLR, but this is a long road...
See these other articles
How to load CLR into process and Create a C# DLL That Can Be Imported in a Delphi App Using stdcall - Possible?

C# and C++ Library

I was wondering if I can use a library -written in C++- in C#
The problem is that library has its own structures and classes..
Will I be able to use it in C#?
Thanks
EDIT This library is open source..
so if my requirements needs something special in C++ code, I will be able do it...

You cannot directly use C++ classes in managed code. The chief problems are not being able to use the same memory allocator as used by the C++ code and not being able to invoke the constructor and destructor easily. A Microsoft employee posted a blog post to show that it is not impossible. I would not recommend doing this.
COM is a solution but that invariably requires a fairly big rewrite and good COM programming skillz. A managed class wrapper in the C++/CLI language is usually the best solution. You could take a peek at the SWIG tool to consider auto-generating those wrapper classes. Beware however that this tool can easily create more problems than it solves.

There are two ways, both using an Adapter (which maps C++ classes to .NET classes):
C++/CLI
COM
The former avoids going via COM, and much of the C++ code might be able to be just compiled with the correct switches.
Additional: In theory P/Invoke might be possible, but all the C++ semantics would be lost, you would need to handle C++ object lifetime manually (and instance references as IntPtr). Plus of course you would need to call the mangled names...

Another option is to write a managed wrapper in C++/CLI. I prefer that instead of using P/Invoke.

How C++ can import a DLL made in C#?

I have a DLL made in C#, this DLL contains some clases like Creator.
I need to load this DLL and use Creator class in C++ unmanaged,
so Is there some way to create that instance or must I load just the functions exposed?
I need something like this:
CreatorInstance->Init();
Is this posible?

Most of what you need can be found here: http://msdn.microsoft.com/en-us/library/x0w2664k%28VS.80%29.aspx
Primarily, you need to learn about the /clr switch for C++ compilation. Then you need to understand the C++ extensions that Microsoft added to allow for mixed assemblies. (A C++ "pointer" to a managed class would use p^ instead of p*, and so on.)

John Fisher's approach using C++/CLI is by far the easiest means of handling this, but it is not the only means.
The other three options are:
1) Use COM interop to wrap the .NET class via COM
2) You can host the CLR in your native, unmanaged application, and call into it. For details, see this article.
3) You can host the Mono runtime, and use it to call your managed code. For details on this, see this page.
Option 2 and 3 are very similar, but IMO, 3 is easier than 2.

Here is an interesting article on how you should be able to accomplish this without using the /CLR option
http://www.codeproject.com/KB/cs/ManagedCOM.aspx
Works pretty well.

First of all, it is possible and you do not "have" to use CLI or the /clr switch. Using the good old COM architecture you can do it pretty easily http://msdn.microsoft.com/en-us/library/zsfww439.aspx. Understanding the way COM works might be the biggest challenge here, but it's usefull once you know it.

What are the best practices when using SWIG with C#?

Has anybody out there used the SWIG library with C#? If you have, what pitfalls did you find and what is the best way to use the library? I am thinking about using it as a wrapper for a program that was written in C and I want to wrap the header files where I can use them in my .NET application.
Edit: Some clarification on target OS's.
I plan on running the application on Linux and Windows, therefore the reason I am looking into SWIG. P/Invoke is not an option.

For my last project, here's the entire C# SWIG configuration file:
%module mdProject
%{
#include "mdProject.h"
%}
I compiled it in SWIG with:
swig -csharp -c++ -I../../Include mdProject.i
This generated a Project.cxx which I compiled and linked directly into the 'main' DLL, so I didn't need a second C++ 'helper' DLL. SWIG also generated a bunch of C# files which I compiled into a .NET DLL. My other wrappers (Java, PHP, etc) do use a helper DLL.
As #patrick mentioned, SWIG uses P/Invoke, so if you have a problem with that, you'll need to find another solution.
If you use types that stray from the ordinary (voids, structures, etc), you will have to do some extra work to get it right, but for the average API using int's, char*'s etc, it's fine.

I think the mistake the earlier posters did was read the docs and not look at the examples.
A few hours ago I needed to interface some C++ classes to C#. I looked in my Swig dir (I already had it for other work), found the directory Examples/csharp/class, browsed the code, loaded the solution, grokked it, copied it, put in my code, it worked, my job was done.
With that said, generated P/Invoke code isn't a solution for all needs. Depending on your project, it may be just as simple to write some simple API wrappers yourself or write managed C++ (Look up SlimDX for a superb example of this).
For my needs, it was simple and easy - I had mystuff.dll, and now in addition I can ship mystuffnet.dll. I'll agree that the doc is difficult to get into.
Edit: I noticed the OP only mentioned C. For that, you don't really need Swig, just use the usual C#/C DLLImport interop syntax. Swig becomes useful when you want to let C++ classes be invoked from C#.

I did attempt to use SWIG to wrap a project C++ for using in .NET a few years ago.
I didn't get very far as it was a massive giant pain to produce the configuration that SWIG required. At the time I just wanted a solution, not to learn another language/api/etc. SWIG may be easier to use these days, I couldn't tell you.
We ended up using Managed C++ to wrap the C++ project. It worked really well.
If you're just invoking functions straight out of a dll, I'd suggest not worrying about either of the above, and just using P/Invoke