c# - Convert array of structs to IntPtr

Question

I am trying to convert an array of the RECT structure (given below) into an IntPtr, so I can send the pointer using PostMessage to another application.

[StructLayout(LayoutKind.Sequential)]
public struct RECT
{
    public int Left;
    public int Top;
    public int Right;
    public int Bottom;

    // lots of functions snipped here
}

// so we have something to send, in reality I have real data here
// also, the length of the array is not constant
RECT[] foo = new RECT[4]; 
IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(foo[0]) * 4);
Marshal.StructureToPtr(foo, ptr, true); // -- FAILS

This gives an ArgumentException on the last line ("The specified structure must be blittable or have layout information."). I need to somehow get this array of RECTs over to another application using PostMessage, so I really need a pointer to this data.

What are my options here?

UPDATE: This seems to work:

 IntPtr result = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(Win32.RECT)) * foo.Length);
 IntPtr c = new IntPtr(result.ToInt32());
 for (i = 0; i < foo.Length; i++)
 {
     Marshal.StructureToPtr(foo[i], c, true);
     c = new IntPtr(c.ToInt32() + Marshal.SizeOf(typeof(Win32.RECT)));
 }

UPDATED AGAIN to fix what arbiter commented on.

Answer 1

StructureToPtr expects struct object, and foo is not structure it is array, that is why exception occurs.

I can suggest you to write structures in cycle (sadly, StructureToPtr does not have overload with Index):

long LongPtr = ptr.ToInt64(); // Must work both on x86 and x64
for (int I = 0; I < foo.Length; I++)
{
    IntPtr RectPtr = new IntPtr(LongPtr);
    Marshal.StructureToPtr(foo[I], RectPtr, false); // You do not need to erase struct in this case
    LongPtr += Marshal.SizeOf(typeof(Rect));
}

Another option is to write structure as four integers, using Marshal.WriteInt32:

for (int I = 0; I < foo.Length; I++)
{
    int Base = I * sizeof(int) * 4;
    Marshal.WriteInt32(ptr, Base + 0, foo[I].Left);
    Marshal.WriteInt32(ptr, Base + sizeof(int), foo[I].Top);
    Marshal.WriteInt32(ptr, Base + sizeof(int) * 2, foo[I].Right);
    Marshal.WriteInt32(ptr, Base + sizeof(int) * 3, foo[I].Bottom);
}

And the last, you can use unsafe keyword, and work with pointers directly.