How to marshal a variable sized array of structs? C# and C++ interop help

Question

I have the following C++ structs

struct InnerStruct
{
   int A;
   int B;
};

struct OuterStruct
{
   int numberStructs;
   InnerStruct* innerStructs;
};

And a C++ function

OuterStruct getStructs();

How can I marshal this to C#? Where the C# definitions is

struct OuterStruct {
   InnerStruct[] innerStructs;
};

Answer 1

You'll have to do this manually, since there's no way to tell the P/Invoke layer how much data to marshal from your C++ return value.

struct OuterStruct {
   int numberStructs;
   IntPtr innerStructs;
};

OuterStruct s = getStructs(); // using DllImport
var structSize = Marshal.SizeOf(typeof(InnerStruct));
var innerStructs = new List<InnerStruct>();
var ptr = s.innerStructs;

for (int i = 0; i < s.numberStructs; i++)
{
    innerStructs.Add((InnerStruct)Marshal.PtrToStructure(ptr, 
        typeof(InnerStruct));
    ptr = ptr + structSize;
}

Note that if you want to free the memory for innerStructs from your C# code, you have to use the standard allocator CoTaskMemAlloc in your C++ code--then you can call Marshal.CoTaskMemFree to free innerStructs.