How to import void * C API into C#?

Question

Given this C API declaration how would it be imported to C#?

int _stdcall z4ctyget(CITY_REC *, void *);

I've been able to get this far:

   [DllImport(@"zip4_w32.dll",
        CallingConvention = CallingConvention.StdCall,
        EntryPoint = "z4ctygetSTD",
        ExactSpelling = false)]
    private extern static int z4ctygetSTD(ref CITY_REC args, void * ptr);

Naturally in C# the "void *" doesn't compile.

Some Googling indicates that it should be translated as "object." Which seems like it should work. But others indicate that "Void * is called a function pointer in C/C++ terms which in C# terms is a delegate". That doesn't make a whole lot of sense here as what would it delegate to? Some similar calls for other APIs found through Googling use other functions in the respective API. But in this API no other call would make sense.

The documentation for the call shows an example:

z4ctyget(&city, “00000”);

Which seems to show that even a static value could be passed.

It will compile with object in place of the void *. I don't know whether this is right and I haven't had an opportunity to test it (licensing issue).

Solution 1

For the void* parameter you can just use an IntPtr

  [DllImport(@"zip4_w32.dll",
        CallingConvention = CallingConvention.StdCall,
        EntryPoint = "z4ctygetSTD",
        ExactSpelling = false)]
    private extern static int z4ctygetSTD(ref CITY_REC args, IntPtr ptr);

Solution 2

You can also use void* if you mark your class as unsafe.

It really depends on what the API is looking for in that parameter.

You can add IntPtr or Object* to get past compiler, but you will still need to pass it the correct data when you call it.

Comments

[Erik Forbes]

Darn you Jared, 30 seconds! Darn you straight to heck! -- =P

[JaredPar]

@Erick :) Had few quick draw wins today

[Mike Chess]

The C declaration I showed was pulled straight from the .h file provided by USPS.

[Stephen Martin]

The documentation from 2007 shows my declaration. In any case, the "00000" example shows that the expected input is a string (almost certainly ANSI). So my C# declaration should be what you are looking for.