Brief Example of Calling .dll Functions in C#

I don’t like C#, but the free version of Visual Studio only lets you use the interface builder in C#/.net programs, so here we are. My goal was to cheat and write the interesting parts of the program in C/C++, compiling to a .dll, and calling it from C#.

This has turned out to be an ordeal.

I have a great handle on calling functions in a .dll from Python. The CTypes module is amazing, and incredibly well documented. C# has a module (service?) in the System.Runtime.InteropServices namespace and the Platform Invoke service for “consuming un-managed code”, but it has been a real pain getting it to work.

I’ll admit that 80% of the problem is that I’m still fairly new to C#, but there is no shortage of information online for vanilla C# programming. Interfacing to a .dll seems to be uncommon enough that it’s hard to find exactly what I’m looking for when I run into a use-case that hasn’t been discussed much.

Here’s what I’m getting at. Let’s say I have a function in my .dll as such:

	return 42;

Here’s what the function in C# would look like for interfacing with GetInteger() in the .dll (which I put in a class called InteropStructTestDLLWrapper):

public extern static int GetInteger();

And here’s how to call that in C#:

int int_from_dll = 0;
int_from_dll = InteropStructTestDLLWrapper.GetInteger();

This behaves exactly as you would expect – it returns the number 42. Things start getting weird when pointers are involved.

Function in .dll:

INTEROPSTRUCTTEST_API void PassIntegerPointer(int *i)
	*i = 27;

Function for calling the .dll function in C#:

[DllImport(InteropStructTest.dll, CallingConvention = CallingConvention.Cdecl)]
public extern static void PassIntegerPointer([In, Out]ref int i);

Calling the function in C#:

InteropStructTestDLLWrapper.PassIntegerPointer(ref int_from_dll);

Now you have to deal with [In, Out] and ref, and CallingConvention.Cdecl. Much of this was guess-and-check to get working using information I gleaned from dozens of StackOverflow posts and other blogs. Things start getting extra weird when you want to pass a pointer to a struct or array of structs.

I decided it was best to just start making a sample .dll and .cs program that demonstrated a clear use-case for passing various data types to and from a .dll. Something that I could reference and add to as I learn. So far it has returning integers, passing a pointer to integer to be modified, passing a pointer to a struct to be modified, and passing an array of structs to be modified (which was super hard to find anything online about).

Right now it has examples of all the things I’ll have to do in small side-project I’m working on. I’ll flesh it out as needed.

Hopefully I’ll save someone some time. I’m embarrassed at how much time I burned getting this far!

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