C# Tutorial For Beginners - Fourth tutorial

Fourth tutorial

Congratulation you would soon be able to hack CsGL but there is one last step you should understand : interop (with C code).You will need a C compiler, I advise gcc for windows called MinGW, it's free, it's good, it's GCC!We will create 3 file:

echo.c

#include

#define DLLOBJECT __declspec(dllexport)

DLLOBJECT void writeln(char* s)
{
printf("%s\n", s);
}
echo.cs using System;
using System.Runtime.InteropServices;

namespace HelloUtil
{
public class Echo
{
[DllImport("echo.native.dll", CallingConvention=CallingConvention.Cdecl)]
static extern void writeln(string s);

string myString;

public Echo(string aString)
{
myString = aString;
}

public void Tell()
{
writeln(myString);
}
}
}
hello.cs using System;
using HelloUtil;

public class Hello
{
public static void Main()
{
Echo h = new Echo("Hello my 1st interop code !");
h.Tell();
}
}
Hehe, here you discover a completly new thing, Attribute."[DllImport(.." is an attribute.You could tag any method/field/class with any number of attribute.They generate extra information that could be used by anyone who could understand them.This DllImport attribute is understand by the compiler and told him that the function below is in fact in a DLL whose name is "echo.native.dll". I add a calling convention parameter as the default .NET calling convention is __stdcall whereas, in C, it's __cdecl.By the way, if you look for DllImport in the documentation, look for DllImportAttribute, because you remove "Attribute" to attribute classname when using them, it's like this.
And now let's compile this! > csc /nologo /t:library /out:echo.dll echo.cs
> csc /nologo /out:hello.exe /r:echo.dll hello.cs
>
> rem "if the following line don't work, read bellow.."
> gcc -shared -o echo.native.dll echo.c
> strip echo.native.dll
the 2 last line (the gcc & strip command) are for building the "C-DLL".If they don't work maybe gcc is not in a directory listed in your path environment variable ? check with: %lt; echo %PATH%Well it's probably not,anyway, so type, assumin mingc is in C:\MinGW: set PATH=C:\MinGW;%PATH%And try again... you sure it's not a syntax error ?If it compile test it now: helloGreat isn't it ?
Now I should admit I didn't tell you all the truth. echo.dll and echo.native.dll are not the same kind of DLL. It's not just the language (C / C#) the C one is a plain executable full of, probably, x86 instruction, whereas the C# one is what MS call a portable executable.. anyway they are different.If you install echo.dll in the GAC it wont work because it won't find echo.native.dll except if you put in into the PATH (like C:\Windows\System32).In the same manner when you add the reference in VS.NET echo.native.dll is overlooked and your program won't work....So either put the native one in your path or copy it in the debug/release directory of VS.NET.Or do everything by hand (makefile? build.bat?) and put all your dll in you build directory, and everything work fine..