从 C# 调用 C dll，返回类型略有不同

Question

I've written a dll in C which has functions I can call when referencing the DLL in C#.If I use a basic type like an int it works fine, but I have structs which are slightly different in C# than they are in C due to language differences. Here is an example. This is the function definition in C#:

[DllImport("hello_world_cuda.dll", CharSet = CharSet.Auto)]
public static extern Batch Cut();

And here is it in C:

extern "C" Batch __declspec(dllexport) __stdcall Cut()

You can see the return type Batch is the same, but here is its definition in C#

class Envelope
{
    public byte[] Payload;
    public byte[] Signature;
}

class Batch
{
    public Envelope[] Messages;
    public int MsgCount;
}

And here is the definition in C

struct Envelope
{
public:
    char* Payload;
    char* Signature;
};

struct Batch
{
public:
    Envelope* Messages;
    int MsgCount;
};

How do I overcome these language differences in order to successfully make the DLL call in C#?

Answer 1

You should define Envelope and Batch as structs in C# too, and apply the StructLaylout attribute:

eg:

[StructLayout(LayoutKind.Sequential, Pack=0)]
struct Envelope
{
   ...
}

Answer 2

Pointers in an unmanaged language do not map to managed arrays as you have done, this is why it is complaining. char is (almost always, with very limited exceptions) an 8 bit value that maps well to byte in C# as you've noticed, but you need to make them pointers in the managed struct as well:

unsafe struct Envelope
{
    public byte* Payload;
    public byte* Signature;
}