Write to fixed length array inside a struct

Question

Ok this one might be simple but I don't have any experience with dealing unmanaged memory within C#. I got a structure in my project containing a fixed length array of four bytes:

[StructLayout(LayoutKind.Sequential)]
unsafe struct MessageHeader {
    ...
    public fixed byte Prefix[4];
    ...
}

Now all I want to do is creating an object of type MessageHeader and assign some bytes to the prefix. I tried the following:

MessageHeader x;
unsafe {
    fixed (byte* ptr = x.Prefix) {
        Marshal.Copy(new byte[] { 128 , 64, 128, 64 }, 0, new IntPtr(ptr), 4); 
    }
    ...
}

However I got the following error: "You cannot use a fixed statement to take the address of an already fixed expression".

I tried to use it without fixed:

Marshal.Copy(new byte[] { 128, 64, 128, 64 }, 0, new IntPtr(x.Prefix), 4);

But this gives me the error of a possibly uninitialized struct. Shouldn't that be irrelevant on writing to an arbitrary unmanaged buffer?

hazzik · Accepted Answer · 2013-06-28 10:24:35Z

1

Try this one. The x.Prefix is already pointer and it is already fixed.

MessageHeader x;

// other ways to initialize x:
// var x = default(MessageHeader);
// var x = new MessageHeader();

unsafe
{
    Marshal.Copy(new byte[] { 128, 64, 128, 64 }, 0, new IntPtr(x.Prefix), 4);
}

edited Jun 28, 2013 at 10:24

answered Jun 28, 2013 at 9:55

hazzik

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7 Comments

mbue Over a year ago

Don't see a difference between your code line and my second code line... As I already stated, I get an error about the uninitialized struct

hazzik Over a year ago

You actually write your x initialization as var x = new MessageHeader();

mbue Over a year ago

Oh my bad, it has been an old error message, on rebuild it works. Sorry for that. Anyway I still get the same error message down below the Marshal.Copy (Use of unassigned variable 'x') when I try to pass the struct somewhere into a method. Any ideas?

hazzik Over a year ago

"So new MessageHeader() is needed? Can't it be allocated on stack?" It will be on stack anyway. It is equal to var x = default(MessageHeader);

mbue Over a year ago

Ok, I initialized the struct as shown in your last posting, now everything works fine. Thank you very much for your help :)

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