Pinning an updateble struct before passing to unma

问题:

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 unmanaged code, the unmanaged code copies the pointer and returns immediately. The unmanaged code can access that struct in background, in another thread. I have no control over the unmanaged code that runs in another thread nor the thread itself.

The fixed { } statement can't be used for pinning because it not designed for async unmanaged pinning.

GCHandle can pin only references, so the struct must be boxed to use GCHandle. I tried it, and it works. The main problem with it, is that you can't update the struct from managed code. To update a struct, first of all we need to unbox it, then update, then box again, but... oops ... box again?!? this means the previous pointer in the memory still point to the old non-up-to-date struct, and the new struct have another pointer, and this means that i need to pass new pointer to the unmanaged code... inapplicable in my case.

How can i pin a struct in the memory without fixed { } statement, and so that i can update it from managed code without change it's pointer?

Thanks.

Edit:

Just thought... is there a way to pin the parent object that contains the struct, and then get the pointer of the struct rather than the container object?

回答1:

Is unsafe code an option?

// allocate unmanaged memory
Foo* foo = (Foo*)Marshal.AllocHGlobal(sizeof(Foo));

// initialize struct
foo->bar = 0;

// invoke unmanaged function which remembers foo
UnsafeNativeMethods.Bar(foo);
Console.WriteLine(foo->bar);

// update struct
foo->bar = 10;

// invoke unmanaged function which uses remembered foo
UnsafeNativeMethods.Qux();
Console.WriteLine(foo->bar);

// free unmanaged memory
Marshal.FreeHGlobal((IntPtr)foo);

This compiles and doesn't throw an exception, but I don't have an unmanaged function at hand to test if it works.

From MSDN:

When AllocHGlobal calls LocalAlloc, it passes a LMEM_FIXED flag, which causes the allocated memory to be locked in place. Also, the allocated memory is not zero-filled.

回答2:

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;
  }
}

回答3:

Instead of pinning, you need to use Marshal.StructureToPtr and Marshal.PtrToStructure to marshal the struct into memory that's usable in native code.

回答4:

Struct example:

[StructLayout(LayoutKind.Sequential)]
public struct OVERLAPPED_STRUCT
{
   public IntPtr InternalLow;
   public IntPtr InternalHigh;
   public Int32 OffsetLow;
   public Int32 OffsetHigh;
   public IntPtr EventHandle;
}

How to pin it to the struct and use it:

OVERLAPPED_STRUCT over_lapped = new OVERLAPPED_STRUCT();
// edit struct in managed code
over_lapped.OffsetLow = 100;
IntPtr pinned_overlap_struct = Marshal.AllocHGlobal(Marshal.SizeOf(over_lapped));
Marshal.StructureToPtr(over_lapped, pinned_overlap_struct, true);

// Pass pinned_overlap_struct to your unmanaged code
// pinned_overlap_struct changes ...

// Get resulting new struct
OVERLAPPED_STRUCT nat_ov = (OVERLAPPED_STRUCT)Marshal.PtrToStructure(pinned_overlap_struct, typeof(OVERLAPPED_STRUCT));
// See what new value is
int offset_low = nat_ov.OffsetLow;
// Clean up
Marshal.FreeHGlobal(pinned_overlap_struct);

回答5:

How about having the struct include an ActOnMe() interface and method something like:

delegate void ActByRef<T1,T2>(ref T1 p1, ref T2 p2);
interface IActOnMe<TT> {ActOnMe<T>(ActByRef<TT,T> proc, ref T param);}
struct SuperThing : IActOnMe<SuperThing>
{
  int this;
  int that;
  ...
  void ActOnMe<T>(ActByRef<SuperThing,T>, ref T param)
  {
    proc(ref this, ref param);
  }
}

Because the delegate takes a generic parameter by reference, it should be possible in most cases to avoid the overhead of creating closures by passing a delegate to a static method along with a reference to a struct to carry data to or from that method. Further, casting an already-boxed instance of SuperThing to IActOnMe<SuperThing> and calling ActOnMe<T> on it will expose the fields of that boxed instance for updating, as opposed to creating another copy of them as would occur with a typecast to the struct.