Develop Reference

Creating C++ DLL called from C#

I'm trying to find a tutorial on creating a C++ class in a DLL that can be called from C#. I've found a few tutorials but they either do not compile in VS 2013 Community or are not on point. The C++ object has only a few exported functions (i.e., that are called from the consuming C# app like ptr->StartAction()). It has about 70 procedures that are not exposed, in assembly language which is why I need to use C++. Currently I expose some functions in the DLL via dllimport etc. But C# requires using fixed(...) (to prevent garbage collection) which slows everything down quite a bit (and they are called 1,000,000s of times). Hence the need to move all that into the DLL. So I need a class that I can instantiate in the C# calling program and then call the public methods in the dll. I'm using .NET 4.0. The DLL will only be used by this one C# app.

You can achieve it through the help of COM component and IDL with an output of Interoperability dll where you can use in your c# project. For more details follow this link.

You can use the DllImport attribute of C#. Here is a sample signature:
[DllImport("path to dll", CallingConvention = CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.BStr)]
public static extern string methodName(string parameterName);
C# will call the method with the same name in the DLL.
Its impossible to say what you really need, because we don't have your method declarations.

Calling c dll in C#

Introduction to the problem:
I've got to control a certain device through API provided with a DLL file, LIB file, and c header files whose functions are declared as dllimport.
When I use the API in a C++ project everything worked just fine - I included the headers, lib , dll, and called the functions as declared in the header files.
The problem begins when trying to call those functions from a C#.NET project, using [DllImport] attribute: The functions were declared with the exact name and parameters, and running the code did not throw any exception. And yet the device did not respond at all, like the functions had never been actually called.
How it is declared in the C header:
int __declspec(dllimport) Init_Card_RTx(unsigned short device_num, unsigned short enabled_channels, unsigned short xmt_channels);
How it is declared in C#:
[DllImport(Path, CallingConvention = CallingConvention.Cdecl, EntryPoint = "Init_Card_RTx")]
public static extern int Init_Card_RTx(UInt16 device_num, UInt16 enabled_channels, UInt16 xmt_channels);
The questions:
Is that because the functions in the headers are declared dllimport?
In that case, do I have to wrap the DLL with C++ functions declared as dllexport?
What are the steps required for a C DLL to be accessible from C#?
In the C# project, do I have to include the LIB file as well? not just the DLL file itself?

Is that because the functions in the headers are declared dllimport?
Possibly. The functions need to be exported. Whether they are exported or not depends on how they DLL was compiled by whoever gave it to you. I'd guess that they should be though (or the C++ code would try import them and fail).
The first thing I do when troubleshooting this kind of thing is to load the DLL in Dependency Walker which will show you all the exported functions. If they're not showing up, they're not exported, and C# can't call them. If they are showing up, then you're ok and you don't need to change any of your C code or create any wrapper functions.
In that case, do I have to wrap the DLL with C++ functions declared as dllexport?
No. C# can only call C functions using DllImport. C++ does name mangling when exporting functions, which makes things a mess and generally not workable.
What you need to do is make your functions exported somehow. My guess is that they are already exported, but if not, you could make some wrapper functions like this.
int __declspec(dllexport) Init_Card_RTx(unsigned short device_num, unsigned short enabled_channels, unsigned short xmt_channels) {
// just call the other function here
}
These should make the function exported and you should see it show up in dependency walker.
What are the steps required for a C DLL to be accessible from C#?
The CLR has to be able to find the dll (usually you just put it in the same directory as the C# exe), and the functions have to be exported as C functions. That's pretty much it.
In the C# project, do I have to include the LIB file as well? not just the DLL file itself?
You don't need to include anything in the C# project at all, just the public static extern wrapper functions with their DllImport attributes on them. So long as the CLR can find the dll at runtime, that's all you need. If it can't find it at runtime, you should get an exception when you call the first imported method.
PS: Get Dependency walker. I can't recommend it more highly for this kind of thing

This kind of thing is always a pain in the butt. You have to make sure that the byte structure of the data being passed matches what your DLL is expecting, and exactly. The native structures in C# don't know that they are being passed externally, so you have to structure the code more rigorously.
See here for more info:
http://msdn.microsoft.com/en-us/library/awbckfbz(v=vs.110).aspx

The reason the functions are declared as dllimport is (correct me if I'm wrong) the the DLL is provided by your card vendor, that's ok because dllexport is only needed when building the DLL.
[DllImport(Path, CallingConvention = CallingConvention.Cdecl, EntryPoint = "Init_Card_RTx")]
The first argument Path is there only for ilustration purposes, am I rigth?
Can you open the DLL with depends.exe or PE Explorer(http://www.heaventools.com/download-pe-explorer.htm) and see the DLL's export table?
I'm asking this because the function name maybe decorated, that could happen if the DLL was compiled as C++ instead of plain C.

Try declacring it like this (you only do this once, in a CPP file, NOT a header)
extern "C" __declspec(dllexport) int __stdcall Init_Card_RTx(unsigned short device_num, unsigned short enabled_channels, unsigned short xmt_channels)
{
//
}
and in C#:
[DllImport("MyDll.dll")]
int Init_Card_RTx(ushort device_num, ushort enabled_channels, ushort xmt_channels);
you need to export it and declare with extern "C". __stdcall is convenient as it's the default on the .net side. I have a macro in my C++ project to make sure I type it the same everytime.
And yes, get dependencywalker....

How can I call a function of a C++ DLL that accepts a parameter of type stringstream from C#?

I want to import an unmanaged C++ DLL and call a function that takes stringstream as a parameter. In C#, there is no stringstream class, so can anyone tell me how to call such a function from a C# program?

You should not expose templated objects via a DLL, period.
Templated objects (e.g. almost everything in std::) become inlined. So in this way, your DLL gets its own private copy of the implementation. The module calling your DLL will also get its own private implementation of stringstream. Passing between them means you are inadvertently weaving two unrelated implementations together. For many projects, if you are using the exact same build settings, it's probably no problem.
But even if you use the same compiler, and mix a release DLL with a debug EXE, you will find stack / heap corruption and other harder-to-find problems.
And that's only using your DLL from another unmanaged C++ exe/dll. Crossing then the lines to .NET is even more of a problem.
The solution is to change your DLL's interface to something that plays friendly across DLL bounds. Either COM (you could use IStream for example), or just a C-style interface like the winapi.

If you can modify the C++ dll, export a plain string version. Otherwise you have to build a managed C++ wrapper project, import the other C++ dll, export it as a managed function, and call that from your C# code. C++ interop really sucks.

I'm afraid that you're going to have to create your own StringStream class in C# in order to be able to consume the functions exported from that DLL. As you mentioned, the .NET Framework doesn't provide any similar class out of the box.
The easiest way is probably to wrap the StringBuilder class provided by the .NET Framework such that it can function as a stream. See this blog post for a further explanation and some sample code.
A similar question was also answered in MSDN Magazine: http://msdn.microsoft.com/en-us/magazine/cc163768.aspx. You might find some of the hints and/or sample code given there useful.

You are trying to bind native C++ code to managed code in C#. Best way of doing that in general is to introduce middle layer in managed C++ that will provide interface to calls from C#.

Create an Wrapper dll in c, or c++ that exposes an friendly call to that function. It's the better way.
for example an
BOOL getString(TCHAR * myreturnString, DWORD *size);

Call c++ library from c#

This question might seem a repeat of previous ones. I have read through a series of posts, but not completely clear for my situation.
I have a c++ library which is created using momentics IDE. I have to be able to use this library into a c# project.
Someone had been working on this project before being handed over to me. Currently, there are 2 layers for making this possible. First, a c++ project includes the complete library with a c++ wrapper. This project creates a dll as the output. This c++ dll is then fed to a c# project, which has dllimport calls to the c++ dll. This c# project again creates a dll. Finally, in order to use the library in c# application, I have to include a reference to both of these dlls.
Is this the correct way to get it working? I was thinking probably there should be a way to simplify the process.
Can someone please help me with this question?

Given that you're using a C++ library, I'm assuming it takes advantage of C++ semantics like classes, rather than just exposing procedures. If this is the case, then the way this is typically done is via a manually-created managed C++ interop library.
Basically, you create a managed C++ library in Visual Studio, reference your existing C++ library, and create a managed wrapper around your existing C++ classes. You then reference this (managed) C++ assembly in your C# project and include the original (unmanaged) C++ library in your C# assembly just as a file that gets placed in the build directory.
This is required because there is no way to reference things like C++ classes via P/Invoke (DllImport) calls.
If your base library is just a series of functions, then you can reference that directly in the C# project via P/Invoke functions.
Either way, all of the libraries mentioned above (for the first, the unmanaged C++ library, the managed C++ assembly, and the C# project, or, for the second, the unmanaced C++ library and the C# project) must be included in any project that references them. You cannot statically link the unmanaged library into the managed assembly.

Seems like you have one wrapper too many but perhaps someone is implementing some sort of facade or adding properties or something. The basic connection between managed and unmanaged will be either DllImport of "flat" function calls - not member functions - or C++/CLI code calling member functions. If the wrapper is C++/CLI it's easiest to write (just include the header for the C++ library) and easiest to call (C# code just adds a .NET reference to it and carries on as normal) so it would be my first choice if there's C++ expertise on the project.
It sounds like whoever you were taking over from is doing it the hard way. If there are less than 20 methods, I would suggest starting over.

You can use:
DllImport
class Example
{
// Use DllImport to import the Win32 MessageBox function.
[DllImport("user32.dll", CharSet = CharSet.Unicode)]
public static extern int MessageBox(IntPtr hWnd, String text, String caption, uint type);
static void Main()
{
// Call the MessageBox function using platform invoke.
MessageBox(new IntPtr(0), "Hello World!", "Hello Dialog", 0);
}
}

How to create a .dll in Visual Studio 2008 for use in a C# App?

I have a C++ class I'd like to access from a C# application. I'll need to access the constructor and a single member function. Currently the app accepts data in the form of stl::vectors but I can do some conversion if that's not likely to work?
I've found a few articles online which describe how to call C++ DLLs and some others which describe how to make .dll projects for other purposes. I'm struggling to find a guide to creating them in Visual Studio 2008 for use in a C# app though (there seem to be a few for VS 6.0 but the majority of the options they specify don't seem to appear in the 2008 version).
If anyone has a step-by-step guide or a fairly basic example to get going from, I'd be very grateful.

The easiest way to interoperate between C++ and C# is by using managed C++, or C++/CLI as it is called. In VisualStudio, create a new C++ project of type "CLR Class Library". There is some new syntax for the parts that you want to make available to C#, but you can use regular C++ as usual.
In this example, I'm using std::vector<int> just to show that you can use standard types - however, in an actual application, I'd prefer to use the .NET types where possible (in this case a System::Collections::Generic::List<int>).
#pragma unmanaged
#include <vector>
#pragma managed
public ref class CppClass
{
public:
CppClass() : vectorOfInts_(new std::vector<int>)
{}
// This is a finalizer, run when GC collects the managed object
!CppClass()
{ delete vectorOfInts_; }
void Add(int n)
{ vectorOfInts_->push_back(n); }
private:
std::vector<int>* vectorOfInts_;
};
EDIT: Changed the class to hold the vector by pointer instead of by value.

If such DLL is unmanaged. You will have to use P/invoke. A p/invoke function definition looks like this:
[DllImport("Library_Name.dll", EntryPoint = "function")]
public static extern void function();
If, on the other hand, is a managed (C++/CLI) DLL (or assembly). You can access it by adding a reference to it on your .NET project.
EDIT:
I think I didn't answer your question at all. But to create a managed C++ DLL to be accessed from .NET, create a new project an choose:
Visual C++/CLR/Class Library. And then, just add its output DLL to your C# project.
That'd do it. :)

Do you have an existing C++ exe that you want to re-build as a dll? Or is all you have the C++ class?
If all you have is the C++ class, you might think about building a .Net dll. While you couldn't pass stl types from C# to C++, I am pretty sure you can pass managed .Net types.
If you must use the stl types in the class, you could convert the data in the C++ class, or the calling app.

I've found a tutorial which works. A quick note though, you have to be careful to copy the .dll into the same directory as the executable. I messed around with trying to tell the executable where the DLL was but gave up in the end.

Use of unmanaged C++ Dlls can be very tedious in C#.
An "easier" route is to place it in a COM wrapper.