Question

I would like to do the following:
Call Unamanaged method that returns the array of type MyStruct[] that it allocated.

Example of c code:

MyStruct[] foo(int size)
{
   Mystruct* st = (MyStruct*)malloc(size * sizeof(MyStruct));
   return st;
}

How should Implement c# calling method?

Thank you!

Answer 1

The Marshal class in the System.Runtime.InteropServices namespace has a lot of methods to help you marshal data to and from unmanaged code.

You need to declare your native method:

[DllImport("myclib.dll")]
public static extern IntPtr Foo(Int32 size);

And also your C struct as a managed value type (you can use attributes on the fields to control exactly how they are mapped to native memory when they are marshalled):

[StructLayout(LayoutKind.Sequential)]
struct MyStruct {
  public Char Character;
  public Int32 Number;
}

Then you can use Marshal.PtrToStructure to marshal each element of the array into a managed value:

var n = 12;
var pointer = Foo(n);
var array = new MyStruct[n];
var structSize = Marshal.SizeOf(typeof(MyStruct));
for (var i = 0; i < n; ++i) {
  array[i] = (MyStruct) Marshal.PtrToStructure(pointer, typeof(MyStruct));
  pointer += structSize;
}

Note that you are using malloc to allocate the memory in the C code. C# us unable to free that memory and you will have to provide another method to free the allocated memory.