passing into c code from c# code in visual studio

Question

I am confused on how to reference my c code from my c# code. I have read around and realized that DLL's are useful, but I do not understand how to connect my dll. From my understanding I make the dll from my c code? Please help

C# (visual studio code)

public partial class Form1: Form{
  [DllImport("simple.dll", CallingConvention = CallingConvention.Cdec1)]
  public static extern void pinfo(string str);

  private void button1_Click(object sender, EventArgs e){
      pinfo("yay");
  }
}

C code - simple.c

#include <stdlib.h>
#include <stdio.h>
#include <time.h>
__declspec( dllexport ) void pinfo(char* str){
    printf("You are in the method %s\n, str);
}

int main(void){
}

my question is how do I connect the c code to the c# code so that the line "You are in the method yay" prints out? I do not understand how to link a dll file to visual studio or how to create the dll file. thanks

Check https://stackoverflow.com/questions/11425202/is-it-possible-to-call-a-c-function-from-c-net and https://stackoverflow.com/questions/2874603/is-it-possible-to-embed-c-code-in-a-c-sharp-project — javier_el_bene, May 30 '17 at 20:30
Your code looks fine to me. What happens when you try to run it? (Note that you do need to export the `pinfo` function first). Use `__declspec( dllexport )` — Dai, May 30 '17 at 20:32
@dai I added that line, I am unsure how to run it because I have a visual studio project and then a simple.c file that is not associated with my visual studio project, I am unsure how to link them — J.Doe, May 30 '17 at 20:39
Read the documentation on how to create a dll. Did you do that? — David Heffernan, May 30 '17 at 20:41

score 0 · Answer 1 · edited Jun 20 '20 at 09:12

There are 3 main steps for getting P/Invoke (DllImport) working in .NET:

Export the function from the C/C++ project.

In Visual C++, use the __declspec( dllexport ) and extern "C" modifiers to, respectively, tell the compiler to export the function in the DLL so it is visible by name, and to not mangle the name either - otherwise the compiler will remove the function entirely if it has no callers inside the project ("dead-code removal"). Functions can also be exported by an ordinal number instead, which presumably is not what you're interested in.
Ensure you're building for the same ISA.

When you use P/Invoke in .NET, the DLL you reference will be loaded into the hosting .NET process, so the DLL must be built to the same CPU instruction-set as the current .NET process. .NET projects run in AnyCPU mode by default, but in recent versions of Visual Studio they "prefer" 32-bit (x86) mode instead of 64-bit (x64 / AMD64). To prevent any confusion, decide if you want 32-bit or 64-bit mode and force your .NET project to build only for that ISA and then set your C/C++ project to build for that ISA too.
Ensure the target DLL file is located in the PATH of the .NET executable - typically this means simply placing it in the same bin output directory as the built .NET executable.

Note that as this DLL file is not a .NET assembly there is no concept of a "project reference" or "assembly reference", so you need to configure your project specifically so that Visual Studio (MSBuild) will copy the DLL file to your bin folder - you can do this by adding the DLL file to your .NET project root in Solution Explorer, then selecting the file and open the Properties window and choose "Build action: Content." and "Copy to output directory: Always".

Regarding "linking" - DLLs are not linked at compile-time - the clue is in the name: "dynamic-linking": the linking is performed at runtime by the operating system, not by the compiler.

For more information on P/Invoke, I recommend reading this QA: Why are Cdecl calls often mismatched in the "standard" P/Invoke Convention?

Everything from "Note that your DllImport declaration is incorrect" is wrong. The unmanaged function is cdecl and the default marshaling is fine. — David Heffernan, May 30 '17 at 21:51
No, you are wrong. Default marshaling is to null terminated character array using prevailing character set. Which defaults to ANSI on desktop windows. All documented. You should read the documentation. — David Heffernan, May 30 '17 at 21:54
@DavidHeffernan My mistake. I did further reading and I misread the documentation. I've removed the incorrect assertions from my answer. — Dai, May 30 '17 at 21:59

passing into c code from c# code in visual studio

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