CODEWITHSUNDEEP
c#.netc++-clinativedllimport
mamidon
mamidon

Reputation: 895

C# dllimport'ing complex datatypes across platforms?

So I'm writing a wrapper in C# for a C dll. The problem is several of the functions use complex datatypes e.g.:

ComplexType* CreateComplexType(int a, int b);

Is there a way I can declare a valid C# type such that I can use dllimport? If I were doing a Windows-only solution I'd probably use C++/CLI as a go-between the native complex type and a managed complex type.

I do have access to the source code of the C dll, so would it be possible to instead use an opaque type (e.g. handles)?

Upvotes: 0

Views: 417

Answers (2)

Hans Passant
Hans Passant

Reputation: 941970

Such a function is difficult to call reliably from a C program, it doesn't get better when you pinvoke it. The issue is memory management, that struct needs to be destroyed again. Which requires the calling program to use the exact same memory allocator as the DLL. This rarely turns out well in a C program but you might be lucky that you have the source code for the DLL so you can recompile it and ensure that everybody is using the same shared CRT version.

There is no such luck from C# of course, the pinvoke marshaller will call CoTaskMemFree() to release the struct. Few real C programs use CoTaskMemAlloc() to allocate the struct so that's a silent failure on XP, an AccessViolationException on Vista and higher. Modern Windows versions have a much stricter heap manager that doesn't ignore invalid pointers.

You can declare the return value as IntPtr, that stops the pinvoke marshaller from trying to destroy it. And then manually marshal with Marshal.PtrToStructure(). This doesn't otherwise stop the memory leak, your program will eventually crash with OOM. Usually anyway.

Upvotes: 1

jlew
jlew

Reputation: 10591

Mono has a good documentation page on using P/Invoke in Windows vs. Linux. Specifically, see the section on marshaling, that discusses simple vs. complex types. If you want to get creative, you could serialize your type to some convenient string-based format like JSON or XML and use that as your marshaling mechanism.

Upvotes: 0

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