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Importing C++ custom library functions into C#

I currently have a large C++ library that I need to incorporate into a C# project. I was successful in using DllImport to get access to some functions from my library, like this:
[DllImport("MyLib.dll")]
public static extern int SampleFunction(string param);
However, I can't seem to figure out how to make it work for functions that either have a parameter or return type that is not defined within the dll itself (i.e., the code in the dll imports it from another file), such as this example function that takes in a list:
[DllImport("MyLib.dll")]
public static extern std::vector<MyClass> MyFunction(string param);
For std library functions, I know I can get the dll and import that in, but what about other classes like MyClass in the above example? Do I need to create a managed wrapper class in C++ for each of these classes, or is there another way to do this?

std::vector<MyClass>, or indeed any unmanaged C++ class, cannot be used for interop to C#. Indeed, even if you wished to consume this function from another unmanaged C++ module you'd have to make sure that you were using the same compiler and dynamic runtime as the DLL which exported the function.
You'll need to find some other way to implement this functionality. There are lots of different ways to do that.
You could expose a p/invoke interface to the functionality. That would likely involve declaring a struct to represent the data of your class, and serializing to and from that struct.
You might consider a COM interface but it won't offer great advantages over p/invoke. You'd still need to serialize your class.
A C++/CLI wrapper would be yet another option. You'd wrap the unmanaged class with a managed C++/CLI class, and then consume that from the C#.

If you need a custom class from a native lib, you'll probably need to export a type library or use managed C++ to write a wrapper. What you're effectively trying to do is marshal a piece of memory from native to managed and now have to deal with how the data type is marshaled.
If at all possible, I would actually recommend trying to do away with the marshaling as when it works it's fine, but when it breaks, it can be rage inducing to debug.
Regardless, I'm actually not sure if anything in the STL can be marshaled. At the very least, I have never seen it done. More times, I've had to do a conversion like so: How can I marshall a vector<int> from a C++ dll to a C# application?
If you want to marshal an actual class, the standard way I've done it up until now is through a type library: Import TLB into C#
Or by linking a native lib into a managed C++ class and then referencing the managed C++ dll into the C# project as a shim.
Beyond that, if you can convert the class to strict C-style functions and pass a struct around, that could allow you to just use pinvoke, this is no different from simply exporting a bunch of helper functions except you can marshal a struct representation back and forth: Marshal C++ struct array into C#

Converting a C++ program into C#

I'm working on a project where I'm converting C++ code to C# manually. I have working knowledge of C# but I've never used C++ before.
What I need to know is how to deal with the header files, since C# does't have anything like that. Say I have buffer.h and buffer.cpp, would I just convert them both and include them in the same buffer.cs file?
Is the C++ header file in any way related to an Ada spec file?

The distinction between includes ".h files" and source ".cpp files" is only one of convention. The convention is that declaration (functions, classes, etc) are in .h files which are #included in implementation (definition), or .cpp files. For most cases you're fine in collapsing X.h and X.cpp to a single X.cs file.
That said, you still need to take a look at what is going on in each file. A basic understanding of C++ would go a long way here, and something I strongly recommend you acquire before you get too far into your translation.

It might help you to think of a C++ header file as containing two major types of things: the class definition, which defines its member data and "interface" (not to be confused with a C# interface type) and "other stuff". The class definition part contains method prototypes and class member variables.
The good news concerning the prototypes is that you simply don't need them in C#. Clients of your class receive prototype information from the implementation itself via the assembly for the namespace. The data members are contained within your C# class implementation, typically as private members which are exposed through C# properties.
The "other stuff" mentioned above can include #defines, which you typically want to turn into const definitions in C#. Other things such as enumerations have equivalents in C# which you of course move into the .cs file for your class.

C# code can't "see" the methods in my C++ dll

I have a code written in C++ (that I did not write) and want to use it in C#, so I decided to make a dll and use this class from there.
I have very little knowledge of C++ and am having problems referencing the methods of this class in my C# project.
The C++ code is like this:
#ifndef BeamAn_class
#define BeamAn_class
#define DllExport __declspec( dllexport )
#include <vector>
#include <cmath>
using namespace std;
public class DllExport BeamAn
{
public:
BeamAn();
~BeamAn();
bool SetGeometry(vector<double>); //I didn't put the DllExport here because I already did it for the whole class. It's okay to do this, right?
//other public methods an stuff
private:
//private methods an stuff
}
#endif
In my C# project I added the reference to the C++ dll normally (right click on the project, add reference. The .lib and .h files are in the same folder of the dll).
But looks like Visual Studio can't "see" the methods of my class. I can create a object of the BeamAn type, but can't use any of its methods.
For example, I can do this:
BeamAn contBeam = new BeamAn();
But can't use any of the methods or atributes inside the class, like this: contBeam.SetLoadFactors(1.0,1.2);
Visual Studio says that "BeamAn does not contain a definition for 'SetLoadFactors' and no extension method 'SetLoadFactors' accepting a first argument type 'BeamAn' could be found (are you missing a using directive or an assembly reference?)
Is there more something I should write in the C++ code to make the dll work properly, or am I doing something wrong when referencing it? I know that I'd have to use "DllImport" in my C# code if I was explicit linking, but that's not what I want to do.
Thank you very much!

There are several ways to make C++ library available in C#.
Use PInvoke - directly call your C++ code. See this article for more information. Also see pinvoke.net for general reference.
Use an interop language to create a managed wrapper around your unmanaged code. If you're in the Microsoft world I would recommend C++/CLI to create mixed mode libraries (dlls that contain both managed and unmanaged code). Here is a brief (and old) article introducing C++/CLI.
Make your C++ COM accessible. Since the code isn't yours this one might not be useful to you. See this post for more information about C++ and COM.
For simple scenarios PInvoke is probably the easiest. For more complicated interactions of the managed and unmanaged code I would recommend writing a mixed mode dll with an interop language. And if those don't work for you, use COM.
To address the error you receive: you can't add a reference to a C++ library from a C# project - they are completely different languages with different memory management systems, different type systems, different everything (well, a lot of things). Also, you can't pass data structures (like vectors, maps or sets) from unmanaged code to managed code.

to use c++ code in a c# project, you have 2 options:
wrap your c++ class in a managed c++ class
use a library called swig
a link that might interest you: Wrapping Visual C++ in C#

Using a C++ dll in C#

I'm attempting to consume a dll written in C++ from a C# application. I have 3rd party source code for the C++ dll (the Cyclone physics engine) and do not want to manually port it over to C#.
In the C++ project
I changed it to output a dll. I changed it to use the /clr flag. I changed it to use Multi-threaded Debug DLL (/MDd) because that was the only one compatible with /clr and also compiled.
In the C# project
I added a reference to the dll. I'm using the cyclone namespace.
At first there was absolutely nothing under the namespace. I think this is because in the C++ code, all classes were declared with no access modifiers and the default is private. So for the class "Particle" I changed the definition to:
public class Particle
{
//...
}
Now I can successfully declare a variable of type Particle from the C# code. However, intellesense and the object browser report Particle to be a struct (?) and it doesn't contain any methods at all. The c++ code declares a bunch of methods after "public:" access modifiers so I don't know what the problem is.
For example,
public:
void integrate(real duration);
What am I doing wrong?

The Particle class is not a managed class, hence it is treated as a struct. You need to use the ref keyword to make it managed and garbage collected. You also need to do the same to every other class that references it which might be a problem. The best solution I think, is to create a managed wrapper class that uses the Particle class internally. This wrapper class can then be referenced by .net.
See here:

I have not actually run into your exact problem before, but I have used C++ dlls in c# before. Typically when calling a dll like that you would use the dllImport keyword. Basically you can define a class that imports all the types and methods from the c++ dll. You can then call those wrapper classes.
I am far from an expert on it, but I have used it to get access to win32 methods and some other libraries that I needed to use.
This link on codeplex has a few links tools that might help. But the most important is probably the Interop Assistant. It can generate the C# wrappers for your c++ dll.
This isn't exactly what you are asking, but I thought it might help to look at a different direction.

Try declaring the c++ class with the ref keyword:
public ref class Particle

Do you have a reference to COM Interop?

Problem in accessing members of class in C# DLL from C++ project

I have added a C# DLL into a C++ project as mentioned at MS support, however I was not able to access its variables and methods inside the class. It also says that it's a struct and not a class, I don't know if it is important but I thought I should mention it is as well. Whenever I write . or -> or :: after the object, nothing appear. But it appear at the name of the class only although they are not static.

Starting with Visual Studio 2005, you can use C++/CLI, Microsoft's ECMA-approved C++ dialect that allows using managed and unmanaged code together. In VS2005, there are the "Managed Extensions for C++", with which you can achieve roughly the same, but you have to use horribly-looking syntaxes for writing managed code in C++ (with lots of double underscores).
With C++/CLI, you can mix managed and unmanaged code in your project, and use C# types directly. IMHO, that's a lot easier than using COM.

To increase compatibility I don't export a class from my DLLs (C# or C++). Instead I expose the class's functions, but the first parameter of each function is a pointer to the class itself. Of you could you also need to expose a: void* CreateMyClassInstance(), and a: DestroyMyClassInstance(void* pInstance).

Read the article posted on below link
http://www.codeproject.com/KB/cs/InterOp.aspx
I think it would help you out.