Wrap a struct containing a fixed size C array in C#

Question

I'm currently stuck wrapping a library that I cannot change due to a struct containing a fixed size unsigned short array, that I seem unable to wrap into something similar in C# using the wrapper generator SWIG.

I have created a minimalistic example swig project file that contains everything needed to reproduce my problem.

%module SwigExperimentWrapper

struct ImportantData
{
   static const unsigned short arraySize = 33; 
   unsigned short someUShortArray[arraySize];
};

The resulting type for the array is not satisfactory. I was expecting something like a

fixed ushort someUShortArray[arraySize];

in C#, but I only get a SWIGTYPE_p_unsigned_short type, which indicates that SWIG probably doesn't have enough information on what to do with this array. Note, that the behaviour is the same if I just define the array without a struct.

I suspect that I need some typemaps for the solution, but first I need to understand, why it won't generate something that matches my expectation. I feel, I am missing something, but I had problems like this with SWIG before, and the problems seemed more relatable.

I'd appreciate any kind of input, as I have not been able to find anything on various searches.

Answer 1

The fixed keyword in this meaning, is only useful when composing a struct

internal unsafe struct Buffer
{
    public fixed char fixedBuffer[128];
}

This defines the array is embedded in the structure, not a referenced array-variable. Of course this must be of fixed size, to define the size of the struct at compile time.

This sample is taken from here. https://docs.microsoft.com/de-de/dotnet/csharp/programming-guide/unsafe-code-pointers/fixed-size-buffers

With the StructLayout Attribute you have control, on which byte a certain member of structure hast to start, and they can even overlap. So you can produced anything you like to Map any bytewise defined record, to a C# structure.

I don't know about your swig tool.

This solution requires unsafe code. (It's just a keyword, nothing to worry about)

An older method for chars would be something like this:

struct Foo
{
    [MarshalAs(UnmanagedType.ByValArray, SizeConst=123)]
     public char[] chars;
}