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 mi…

Accepted Answer

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:

- You should add a field to your structure that includes a count of the array elements (this will make your life much easier)

- Declare the field in C# as: `IntPtr data;` with no attributes.

- Use `Marshal.SizeOf(typeof(A))` to get the size of the struct in unmanaged memory.

- Use `Marshal.PtrToStructure` to convert a single unmanaged structure to C#

- Use `IntPtr.Add(ptr, sizeofA)` to move to the next structure in the array

- Loop until you run out.

How use pinvoke for C struct array pointer to C#

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