C# 中的编组结构

Question

I have this piece of code and it generates an error:

    [StructLayout(LayoutKind.Sequential, Pack = 1)]
    public struct MyItem
    {
        [MarshalAs(UnmanagedType.LPWStr)]
        public string Name;
        public int ID;
        public double ID1;

    }

    private void button1_Click(object sender, RoutedEventArgs e)
    {

        MyItem[] items = new MyItem[6];
        items[0].Name = "JFK";
        items[0].ID = 35;
        items[1].Name = "LBJ";
        items[1].ID = 36;
        items[2].Name = "Tricky Dicky";
        items[2].ID = 37;
        items[3].Name = "Gerald Ford";
        items[3].ID = 38;
        items[4].Name = "Jimmy Carter";
        items[4].ID = 39;
        items[5].Name = "Ronald Reagan";
        items[5].ID = 40;

        IntPtr itemsPtr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(MyItem)) * 
           items.Length);
        try
        {
            IntPtr item = new IntPtr(itemsPtr.ToInt32());
            for (int i = 0; i < items.Length; i++)
            {
                Marshal.StructureToPtr(items[i], item, true);
                item = new IntPtr(item.ToInt32() + Marshal.SizeOf(typeof(MyItem)));
            }         
        }
        finally
        {
            Marshal.FreeHGlobal(itemsPtr);
        }

When I run this code, I am getting write protection error in Marshal.StructureToPtr(items[i], item, true);

What is the problem and how do I solve it?

Answer 1

You should be passing false to the fDeleteOld parameter of StructureToPtr() .

By passing true you are asking the marshaller to delete the contents of item . Since you are filling this out for the first time, this results in a memory access failure because the memory is not yet valid.

The documentation states:

StructureToPtr copies the contents of structure to the pre-allocated block of memory that the ptr parameter points to. If the fDeleteOld parameter is true, the pre-allocated buffer is deleted with the appropriate deletion method on the embedded pointer, but the buffer must contain valid data .

The emphasis is mine.

Incidentally I think the loop code looks neater like this:

Int32 addr = itemsPtr.ToInt32();
for (int i = 0; i < items.Length; i++)
{
    Marshal.StructureToPtr(items[i], new IntPtr(addr), false);
    addr += Marshal.SizeOf(typeof(MyItem));
}