Calling a Delphi DLL from C# produces unexpected results

Question

I have a Delphi DLL that I did not write, but need to call from a C# ASP.NET 3.5 app. Here is the function definition I got from the developers:

function Cr         


        

Answer 1

The return value might be another problem. It is probably either a memory leak(They allocate a buffer on the heap and never free it) or an access to already free memory(They return a local string variable cast to PChar).

Returning strings(or variable sized data in general) from a function to another module is problematic in general.

One solution(used by winapi) is to require the caller to pass in a buffer and its size. The disadvantage of that is that if the buffer is too small the function fails, and the caller must call it again with a larger buffer.

Another possible solution is to allocate the buffer from the heap in the function and return it. Then you need to export another function which the caller must use to free the allocated memory again. This ensures that the memory is freed by the same runtime which allocated it.

Passing a (Delphi)string parameter between different(not borland) languages is probably impossible. And even between Delphi modules you to ensure both modules use the same instance of the memory manager. Usually this means adding "uses ShareMem" as the first uses to all modules. Another difference is the calling convention "register" which is a fastcall convention, but not identical with the fastcall MS compilers use.

A completely different solution could be recompiling the Delphi dll with one of the Delphi.net compilers. How much work that is depends on their code.