How to Write My C++ Function So I Can Call It from C#

Is it possible to call a C function from C#.Net

The example will be, for Linux:

1) Create a C file, libtest.c with this content:

#include <stdio.h>

void print(const char *message)
{
  printf("%s\\n", message);
}

That’s a simple pseudo-wrapper for printf. But represents any C function in the library you want to call. If you have a C++ function don’t forget to put extern C to avoid mangling the name.

2) create the C# file

using System;

using System.Runtime.InteropServices;

public class Tester
{
        [DllImport("libtest.so", EntryPoint="print")]

        static extern void print(string message);

        public static void Main(string[] args)
        {

                print("Hello World C# => C++");
        }
}

3) Unless you have the library libtest.so in a standard library path like “/usr/lib”, you are likely to see a System.DllNotFoundException, to fix this you can move your libtest.so to /usr/lib, or better yet, just add your CWD to the library path: export LD_LIBRARY_PATH=pwd

credits from here

EDIT

For Windows, it's not much different.
Taking an example from here, you only have yo enclose in your *.cpp file your method with extern "C"
Something like

extern "C"
{
//Note: must use __declspec(dllexport) to make (export) methods as 'public'
      __declspec(dllexport) void DoSomethingInC(unsigned short int ExampleParam, unsigned char AnotherExampleParam)
      {
            printf("You called method DoSomethingInC(), You passed in %d and %c\n\r", ExampleParam, AnotherExampleParam);
      }
}//End 'extern "C"' to prevent name mangling

then, compile, and in your C# file do

[DllImport("C_DLL_with_Csharp.dll", EntryPoint="DoSomethingInC")]

public static extern void DoSomethingInC(ushort ExampleParam, char AnotherExampleParam);

and then just use it:

using System;

    using System.Runtime.InteropServices;

    public class Tester
    {
            [DllImport("C_DLL_with_Csharp.dll", EntryPoint="DoSomethingInC")]

    public static extern void DoSomethingInC(ushort ExampleParam, char AnotherExampleParam);

            public static void Main(string[] args)
            {
                    ushort var1 = 2;
                    char var2 = '';  
                    DoSomethingInC(var1, var2);
            }
    }

How to write my C++ function so I can call it from C#?

I'll give you an example.

You should declare your C++ functions for export like so (assuming recent MSVC compiler):

extern "C"             //No name mangling
__declspec(dllexport)  //Tells the compiler to export the function
int                    //Function return type     
__cdecl                //Specifies calling convention, cdelc is default, 
                       //so this can be omitted 
test(int number){
    return number + 1;
}

And compile your C++ project as a dll library. Set your project target extension to .dll, and Configuration Type to Dynamic Library (.dll).

Then, in C# declare:

public static class NativeTest
{
    private const string DllFilePath = @"c:\pathto\mydllfile.dll";

    [DllImport(DllFilePath , CallingConvention = CallingConvention.Cdecl)]
    private extern static int test(int number);

    public static int Test(int number)
    {
        return test(number);
    }
}

Then you can call your C++ test function, as you would expect. Note that it may get a little tricky once you want to pass strings, arrays, pointers, etc. See for example this SO question.

calling c function from C#

You are looking for P/Invoke.

You do will need a Reference to System.Runtime.InteropServices and then do the following, if your C DLL contains a function called increase_int:

[DllImport("mylib.dll")]
private static extern int increase_int(int in_value);

and use it from your code doing

int newValue = increase_int(oldValue);

Calling C# from C

There is more than just COM interop if you want to call into managed code from C or C++. The are also the following lesser known methods (taken from MSDN FAQ):

How do I call a .NET assembly from native Visual C++?

There are basically four methods to
call .NET assembly from native VC++ code:

1. CLR Hosting API: Native VC++ module calls CLR Hosting APIs to host CLR, load and call the .NET assembly (sample code: CppHostCLR).

2. COM Interop: If the .NET assembly can be exposed as a COM component, native VC++ module can call into the .NET assembly through .NET – COM interop (sample code: CppCOMClient).

3. Reverse PInvoke: The managed code calls native code passing a delegate that the native code can call back (sample code: CSPInvokeDll).

4. C++/CLI: If the module containing native VC++ code is allowed to enable CLR, the native VC++ code can call
   .NET assembly directly (sample code: Consuming C# Library in native C or C++ using C++/CLI)

Calling C function (Linux) from C# (Windows)

does it matter if my .so file was created in a Linux environment ?

Yes, it matters.

Shared Object (so) files are generated by linkers and contain code. They are platform specific. Even two versions of Linux running on two different CPUs are quite likely to be unable to use the same .so file.

Can I just copy the .so file to a Windows environment and have C# use it without any issues ?

A Windows OS will not use an .so file. It requires .dll files and they are a completely different format ( and in general require completely different code to do the same job ).

I believe it shouldn't matter because .so is a dynamic library and is filled with symbols not dependent on the platform it's created on.

It is not simply a file full of symbols. It is code that will be executed as well. And that code is platform specific.

Using C functions in C# Program

So you have a bunch of C files lying around your project. They contain functions you want to call from C# code. Fair enough, sure, so let's count the possible ways you can achieve that.

1. Rewrite those functions into C#. This might actually be the most sensible option if those functions aren't being used elsewhere (like in a separate C project, for example), because then you don't have to maintain code in two separate languages for the same project. C# also has many advantages over C, like safety and expressiveness, and depending on what those functions are doing, you might not even see a significant performance difference.
2. Put that code in a native DLL, export those functions, and use P/Invoke from C# to call them. This is what involves the least modification to the original code.
3. Write a C++/CLI wrapper that calls those functions, and call the C++/CLI code from C#. This is especially useful if you want to present an OOP-style interface to the C# code.
4. Use a C interpreter to execute those C files like they were scripts. This will probably be slower, more brittle, harder to maintain, and there might be other restrictions depending on which interpreter you use and what your C code looks like.

How to set up a C++ function so that it can be used by p/invoke?

You'll want to use extern "C" as well as __declspec(export), like so:

extern "C" _declspec(dllexport)  bool TestFunc()
{
    return true;
}

For full details, see MSDN on Marshalling Types.

Calling C# code from C++, but ExecuteInDefaultAppDomain() is too limited

Compile your C++ code with the /clr flag. With that, you can call into any .NET code with relative ease.

For example:

#include <tchar.h>
#include <stdio.h>

int _tmain(int argc, _TCHAR* argv[])
{
    System::DateTime now = System::DateTime::Now;
    printf("%d:%d:%d\n", now.Hour, now.Minute, now.Second);

    return 0;
}

Does this count as "C++"? Well, it's obviously not Standard C++ ...

Calling C++ function from C#, with lots of complicated input and output parameters

Since there seems to be some interest in using It Just Works (IJW) with C++/CLI, I'll post some info about that, further google searches and research will need to be done to figure it all. C++/CLI can be enabled with a single compiler flag (/CLI, enabled through Property Page->General->Common Language Runtime Support). C++/cli is NOT c++, but rather just another managed language. C++/CLI classes can be compiled into dll's and called directly from other .NET projects (C#, VB.NET, ect). However, unlike the other .NET languages it can directly interact with C++ code.

This is an ok start to learning C++/CLI. The big thing to learn is the decorations that tell you the class is managed (.NET class) and not Vanila C++. The "ref" keyword decalres the definition as a .NET definition:

public ref class Foo{ public: void bar();};//Managed class, visible to C#
public ref struct Foo{};//Managed struct, visible to C#

All reference classes are referred to with Handles rather than pointers or references. A handle is denoted by the ^ operator. To make a new handle, you use gcnew, and to access functions/members of the handle, use the -> operator.

//in main
Foo^ a = gcnew Foo();
a->bar();

You often have to move structures common from C# to native types and back again. (such as managed Array^ or String^ to void* or std::string). This process is called Marshaling. This handy table is pretty useful for figuring that out.

A common task is to create a wrapper for a native class, done like this:

//Foo.h
#include <string>
namespace nativeFoo
{
    class Foo
    {
     private:
        std::string fooHeader;
     public:
        Foo() {fooHeader = "asdf";}
        std::string Bar(std::string& b) {return fooHeader+b;}
    };
}
//ManagedFoo.h
#include "foo.h"
namespace managedFoo
{
    public ref class Foo
    {
        private:
             nativeFoo::Foo* _ptr;
        public:
             Foo(){_ptr = new nativeFoo::Foo();}
             ~Foo(){!Foo();}
             !Foo(){if (_ptr){delete ptr;ptr = NULL;}}

             String^ bar(String^ b)
             {
                 return marshal_as<String^>(_ptr->bar(marshal_as<std::string>(b)));
             }
    };
}

Warning: I am totally missing a bunch of #include and #using statements, this is just to give a general gist of how to use this.

Calling a function from a string in C#

Yes. You can use reflection. Something like this:

Type thisType = this.GetType();
MethodInfo theMethod = thisType.GetMethod(TheCommandString);
theMethod.Invoke(this, userParameters);

With the above code, the method which is invoked must have access modifier public. If calling a non-public method, one needs to use the BindingFlags parameter, e.g. BindingFlags.NonPublic | BindingFlags.Instance:

Type thisType = this.GetType();
MethodInfo theMethod = thisType
    .GetMethod(TheCommandString, BindingFlags.NonPublic | BindingFlags.Instance);
theMethod.Invoke(this, userParameters);

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