GCHandle. How to pin object that contain fields defined as class

Question

I want to pin some managed object in .Net for coping it data into byte array. For pinning and coping I use next code:

C c = new C();
byte[] b = new byte[Marshal.SizeOf(c)];
GCHandle gch = GCHandle.Alloc(c, GCHandleType.Pinned);
Marshal.Copy(gch.AddrOfPinnedObject(), b, 0, b.Length);
gch.Free();

And when I declare my object definitions as folow:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
struct A
{
    public int a;
}
[StructLayout(LayoutKind.Sequential, Pack = 1)]
struct C
{
    public A a0;
    public A a1;
    public A a2;
}

All work fine. When I declare my object definitions as folow:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
struct A
{
    public int a;
}
[StructLayout(LayoutKind.Sequential, Pack = 1)]
class C
{
    public A a0;
    public A a1;
    public A a2;
}

Also all work fine. But when I declare both my objects as class:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
class A
{
    public int a;
}
[StructLayout(LayoutKind.Sequential, Pack = 1)]
class C
{
    public A a0;
    public A a1;
    public A a2;
}

then ArgumentException is thrown in GCHandle.Alloc(...) 'Object contains non-primitive or non-blittable data.'

Why when A is defined as struct all work fine. But when as class is not work? It is possible to make this work with both types A and C defined as classes?

[This](http://stackoverflow.com/questions/15544818/non-blitable-error-on-a-blitable-type) may help — Yuval Itzchakov, Jan 22 '16 at 08:19

score 3 · Answer 1 · answered Jan 22 '16 at 08:42

You are trying to defeat the garbage collector, you are copying a reference to a class object into a blob of memory that the GC cannot see. That is not valid, the value of such a reference turns into junk in as little as a nanosecond later. Whenever the GC runs, you don't know when. Nothing the GC can do to update the reference, it does not know it exists.

Doing this can only be correct when the objects that those references point to cannot be garbage collected and cannot be moved. In other words, they have to be pinned. You convince the marshaller that you might get this correct with:

[StructLayout(LayoutKind.Sequential, Pack = 1)]
class C
{
    public IntPtr a0;
    public IntPtr a1;
    public IntPtr a2;
}

You already know how to get those IntPtr values. With the additional stipulation, not checked by the CLR, that you keep those objects pinned as long as other code might be using the blob of memory.

But when I declare an class C as you propose I cannot create byte[] as in my code example. Marshal.SizeOf for your class C returns 12 or 24 (32 or 64 bit) and Marshal.Copy copies into byte[] not an data from A class members but IntPtr values splitted into bytes? — Andrew D., Jan 22 '16 at 10:20
12 is correct, 3 object references in 32-bit mode requires 3 x 4 = 12 bytes. And 3 x 8 = 24 bytes in 64-bit mode. And of course it doesn't copy members, it copies the reference. If you want to copy members as well then you must declare it as a *struct*. Avoid asking an XY question, nobody knows why you are trying to do this. — Hans Passant, Jan 22 '16 at 10:23

GCHandle. How to pin object that contain fields defined as class

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