Pinning an updateble struct before passing to unmanaged code?

Question

I using some old API and need to pass the a pointer of a struct to unmanaged code that runs asynchronous.

In other words, after i passing the struct pointer to the unman

Answer 1

Using pinned memory in this case is not a good idea, given that the memory for the struct needs to be valid for a long time. GCHandle.Alloc() will box the structure and store it on the heap. With it being pinned, it will be a long term burden to the garbage collector as it needs to constantly find a way around the rock in the road.

The simple solution is to allocate memory for the struct in unmanaged memory. Use Marshal.SizeOf() to get the size of the structure and Marshal.AllocCoTaskMem() to allocate the memory. That gets you the pointer you need to pass to the unmanaged code. Initialize the memory with Marshal.StructureToPtr(). And read updates to the structure written by the unmanaged code with PtrToStructure().

If you do this frequently, you'll be constantly copying the structure. That could be expensive, depending on the size of the structure. To avoid that, use an unsafe pointer to access the unmanaged memory directly. Some basic syntax:

using System;
using System.Runtime.InteropServices;

class Program {
  unsafe static void Main(string[] args) {
    int len = Marshal.SizeOf(typeof(Test));
    IntPtr mem = Marshal.AllocCoTaskMem(len);
    Test* ptr = (Test*)mem;
    ptr->member1 = 42;
    // call method
    //..
    int value = ptr->member1;
    Marshal.FreeCoTaskMem(mem);
  }
  public struct Test {
    public int member1;
  }
}