Why does LayoutKind.Sequential work differently if a struct contains a DateTime field?

Question

Why does LayoutKind.Sequential work differently if a struct contains a DateTime field?

Consider the following code (a console app which must be compiled with "unsafe" enabled):

using System;
using System.Runtime.InteropServices;

namespace ConsoleApplication3
{
    static class Program
    {
        static void Main()
        {
            Inner test = new Inner();

            unsafe
            {
                Console.WriteLine("Address of struct   = " + ((int)&test).ToString("X"));
                Console.WriteLine("Address of First    = " + ((int)&test.First).ToString("X"));
                Console.WriteLine("Address of NotFirst = " + ((int)&test.NotFirst).ToString("X"));
            }
        }
    }

    [StructLayout(LayoutKind.Sequential)]
    public struct Inner
    {
        public byte First;
        public double NotFirst;
        public DateTime WTF;
    }
}

Now if I run the code above, I get output similar to the following:

Address of struct = 40F2CC
Address of First = 40F2D4
Address of NotFirst = 40F2CC

Note that the address of First is NOT the same as the address of the struct; however, the address of NotFirst is the same as the address of the struct.

Now comment out the "DateTime WTF" field in the struct, and run it again. This time, I get output similar to this:

Address of struct = 15F2E0
Address of First = 15F2E0
Address of NotFirst = 15F2E8

Now "First" does have the same address as the struct.

I find this behaviour surprising given the use of LayoutKind.Sequential. Can anyone provide an explanation? Does this behaviour have any ramifications when doing interop with C/C++ structs that use the Com DATETIME type?

[EDIT] NOTE: I have verified that when you use Marshal.StructureToPtr() to marshal the struct, the data is marshalled in the correct order, with the "First" field being first. This seems to suggest that it will work fine with interop. The mystery is why the internal layout changes - but of course, the internal layout is never specified, so the compiler can do what it likes.

[EDIT2] Removed "unsafe" from struct declaration (it was leftover from some testing I was doing).

[EDIT3] The original source for this question was from the MSDN C# forums:

http://social.msdn.microsoft.com/Forums/en-US/csharplanguage/thread/fb84bf1d-d9b3-4e91-823e-988257504b30

Answer 1

Why does LayoutKind.Sequential work differently if a struct contains a DateTime field?

It is related to the (surprising) fact that DateTime itself has layout "Auto" (link to SO question by myself). This code reproduces the behavior you saw:

static class Program
{
    static unsafe void Main()
    {
        Console.WriteLine("64-bit: {0}", Environment.Is64BitProcess);
        Console.WriteLine("Layout of OneField: {0}", typeof(OneField).StructLayoutAttribute.Value);
        Console.WriteLine("Layout of Composite: {0}", typeof(Composite).StructLayoutAttribute.Value);
        Console.WriteLine("Size of Composite: {0}", sizeof(Composite));
        var local = default(Composite);
        Console.WriteLine("L: {0:X}", (long)(&(local.L)));
        Console.WriteLine("M: {0:X}", (long)(&(local.M)));
        Console.WriteLine("N: {0:X}", (long)(&(local.N)));
    }
}

[StructLayout(LayoutKind.Auto)]  // also try removing this attribute
struct OneField
{
    public long X;
}

struct Composite   // has layout Sequential
{
    public byte L;
    public double M;
    public OneField N;
}

Sample output:

64-bit: True
Layout of OneField: Auto
Layout of Composite: Sequential
Size of Composite: 24
L: 48F050
M: 48F048
N: 48F058

If we remove the attribute from OneField, things behave as expected. Example:

64-bit: True
Layout of OneField: Sequential
Layout of Composite: Sequential
Size of Composite: 24
L: 48F048
M: 48F050
N: 48F058

These example are with x64 platform compilation (so the size 24, three times eight, is unsurprising), but also with x86 we see the same "disordered" pointer addresses.

So I guess I can conclude that the layout of OneField (resp. DateTime in your example) has influence on the layout of the struct containing a OneField member even if that composite struct itself has layout Sequential. I am not sure if this is problematic (or even required).

---

According to comment by Hans Passant in the other thread, it no longer makes an attempt to keep it sequential when one of the members is an Auto layout struct.

Answer 2

Go read the specification for layout rules more carefully. Layout rules only govern the layout when the object is exposed in unmanaged memory. This means that the compiler is free to place the fields however it wants until the object is actually exported. Somewhat to my surprise, this is even true for FixedLayout!

Ian Ringrose is right about compiler efficiency issues, and that does account for the final layout that is being selected here, but it has nothing to do with why the compiler is ignoring your layout specification.

A couple of people have pointed out that DateTime has Auto layout. That is the ultimate source of your surprise, but the reason is a bit obscure. The documentation for Auto layout says that "objects defined with [Auto] layout cannot be exposed outside of managed code. Attempting to do so generates an exception." Also note that DateTime is a value type. By incorporating a value type having Auto layout into your structure, you inadvertently promised that you would never expose the containing structure to unmanaged code (because doing so would expose the DateTime, and that would generate an exception). Since the layout rules only govern objects in unmanaged memory, and your object can never be exposed to unmanaged memory, the compiler is not constrained in its choice of layout and is free to do whatever it wants. In this case it is reverting to the Auto layout policy in order to achieve better structure packing and alignment.

There! Wasn't that obvious!

All of this, by the way, is recognizable at static compile time. In fact, the compiler is recognizing it in order to decide that it can ignore your layout directive. Having recognized it, a warning here from the compiler would seem to be in order. You haven't actually done anything wrong, but it's helpful to be told when you've written something that has no effect.

The various comments here recommending Fixed layout are generally good advice, but in this case that wouldn't necessarily have any effect, because including the DateTime field exempted the compiler from honoring layout at all. Worse: the compiler isn't required to honor layout, but it is free to honor layout. Which means that successive versions of CLR are free to behave differently on this.

The treatment of layout, in my view, is a design flaw in CLI. When the user specifies a layout, the compiler shouldn't go lawyering around them. Better to keep things simple and have the compiler do what it is told. Especially so where layout is concerned. "Clever", as we all know, is a four letter word.

Answer 3

A few factors

• doubles are a lot faster if they are aligned
• CPU caches may work better if there are no “holes” in the struck

So the C# compiler has a few undocumented rules it uses to try to get the “best” layout of structs, these rules may take into account the total size of a struct, and/or if it contains another struct etc. If you need to know the layout of a struct then you should specify it yourself rather than letting the compiler decide.

However the LayoutKind.Sequential does stop the compiler changing the order of the fields.