How use pinvoke for C struct array pointer to C#

Question

I am trying to use pinvoke to marshall an array of structures inside another structure from C to C#. AFAIK, no can do.
So instead, in the C structure I declare a ptr to my array, and malloc. Problems: 1) How do I declare the equivalent on the C# side? 2) How do I allocate and use the equivalent on the C# side?

//The C code
typedef struct {
       int a;
       int b; } A;
typedef struct {
      int c;
      // A myStruct[100];    // can't do this, so:
      A *myStruct; } B;

//The c# code:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public class A{
    int a;
    int b;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public class B{
      int c;
      // can't declare array of [100] A structures...
     ?
 }

[EDIT]: Somehow I misinterpreted what I read elsewhere about fixed array of objects on the c# side. And I can fix the array size in C So compiled ok, but then I get "object reference not set to an instance of an object" when using:

data.B[3].a = 4567; So, reading elsewhere about what this error might be, I added this method:

public void initA()
        {
          for (int i = 0; i < 100; i++) { B[i] = new A(); }
        }

Again, compiled OK, but same error msg.

Answer 1

To marshal "complex" structures like this between C and C#, you have a couple of options.

In this case, I would highly recommend that you try to embed a fixed array into your C-side structure, as it will simplify the C# side a lot. You can use the MarshalAs attribute to tell C# how much space it needs to allocate in the array at run-time:

// In C:
typedef struct
{
 int a;
 int b;
} A;

typedef struct
{
 int c;
 A data[100];
} B;

// In C#:
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct A 
{
  int a;
  int b;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct B
{
  int c;
  [MarshalAs(UnmanagedType.LPArray, SizeConst=100)]
  A[] data = new data[100];
}

If you don't know, or can't specify, a fixed size for your array, then you'll need to do what you did and declare it as a pointer in C. In this case, you cannot tell C# how much memory the array is going to use at run-time, so you're pretty much stuck with doing all of the marshalling by hand. This question has a good rundown of how that works, but the basic idea is:

1. You should add a field to your structure that includes a count of the array elements (this will make your life much easier)
2. Declare the field in C# as: IntPtr data; with no attributes.
3. Use Marshal.SizeOf(typeof(A)) to get the size of the struct in unmanaged memory.
4. Use Marshal.PtrToStructure to convert a single unmanaged structure to C#
5. Use IntPtr.Add(ptr, sizeofA) to move to the next structure in the array
6. Loop until you run out.