Why does an empty struct in C# consume memory

问题

I always understood structs (value types) contain exactly the number of bytes as defined in the fields of the structure... however, I did some tests and there seems to be an exception for the empty structs:

public class EmptyStructTest
{
    static void Main(string[] args)
    {
        FindMemoryLoad<FooStruct>((id) => new FooStruct());
        FindMemoryLoad<Bar<FooStruct>>((id) => new Bar<FooStruct>(id));
        FindMemoryLoad<Bar<int>>((id) => new Bar<int>(id));
        FindMemoryLoad<int>((id) => id);
        Console.ReadLine();
    }

    private static void FindMemoryLoad<T>(Func<int, T> creator) where T : new()
    {
        GC.Collect(GC.MaxGeneration);
        GC.WaitForFullGCComplete();
        Thread.MemoryBarrier();
        long start = GC.GetTotalMemory(true);

        T[] ids = new T[10000];
        for (int i = 0; i < ids.Length; ++i)
        {
            ids[i] = creator(i);
        }

        long end = GC.GetTotalMemory(true);
        GC.Collect(GC.MaxGeneration);
        GC.WaitForFullGCComplete();
        Thread.MemoryBarrier();

        Console.WriteLine("{0} {1}", ((double)end-start) / 10000.0, ids.Length);
    }

    public struct FooStruct { }

    public struct Bar<T> where T : struct
    {
        public Bar(int id) { value = id; thing = default(T); }

        public int value;
        public T thing;
    }
}

If you run the program, you'll find that en FooStruct which has obviously 0 bytes of data will consume 1 byte of memory. The reason this is a problem for me is that I want Bar<FooStruct> to consume exactly 4 bytes (because I'm going to allocate it a lot).

Why does it have this behavior and is there a way to fix this (e.g. is there a special thing that consumes 0 bytes-- I'm not looking for a redesign)?

回答1:

Summary: An empty struct in .NET consumes 1 byte. You can think of this as packing, since the unnamed byte is only accessible via unsafe code.

More information: if you do all your pointer arithmetic according to values reported by .NET, things work out consistently.

The following example illustrates using adjacent 0-byte structures on the stack, but these observations obviously apply to arrays of 0-byte structures as well.

struct z { };

unsafe static void foo()
{
    var z3 = default(z);
    bool _;
    long cb_pack, Δz, cb_raw;
    var z2 = default(z);    // (reversed since stack offsets are negative)
    var z1 = default(z);
    var z0 = default(z);

    // stack packing differs between x64 and x86
    cb_pack = (long)&z1 - (long)&z0; // --> 1 on x64, 4 on x86

    // pointer arithmetic should give packing in units of z-size
    Δz = &z1 - &z0; // --> 1 on x64, 4 on x86

    // if one asks for the value of such a 'z-size'...
    cb_raw = Marshal.SizeOf(typeof(z));     // --> 1

    // ...then the claim holds up:
    _ = cb_pack == Δz * cb_raw;     // --> true

    // so you cannot rely on special knowledge that cb_pack==0 or cb_raw==0
    _ = &z0 /* + 0 */ == &z1;   // --> false
    _ = &z0 /* + 0 + 0 */ == &z2;   // --> false

    // instead, the pointer arithmetic you meant was:
    _ = &z0 + cb_pack == &z1;   // --> true
    _ = &z0 + cb_pack + cb_pack == &z2; // --> true

    // array indexing also works using reported values
    _ = &(&z0)[Δz] == &z1;  // --> true

    // the default structure 'by-value' comparison asserts that
    // all z instances are (globally) equivalent...
    _ = EqualityComparer<z>.Default.Equals(z0, z1); // --> true

    // ...even when there are intervening non-z objects which
    // would prevent putative 'overlaying' of 0-sized structs:
    _ = EqualityComparer<z>.Default.Equals(z0, z3); // --> true

    // same result with boxing/unboxing
    _ = Object.Equals(z0, z3);  // -> true

    // this one is never true for boxed value types
    _ = Object.ReferenceEquals(z0, z0); // -> false
}

As I mentioned in a comment, @supercat got it right when he noted, "There probably wouldn't have been any problem with designing .NET to allow for zero-length structures from the beginning, but there could be some things that would break if it were to start doing so now."

EDIT: If you need to programmatically distinguish between 0-byte vs. 1-byte value types, you can use the following:

public static bool IsZeroSizeStruct(Type t)
{
    return t.IsValueType && !t.IsPrimitive && 
           t.GetFields((BindingFlags)0x34).All(fi => IsZeroSizeStruct(fi.FieldType));
}

Note that this correctly identifies arbitrarily nested structs where the total size would be zero.

[StructLayout(LayoutKind.Sequential)]
struct z { };
[StructLayout(LayoutKind.Sequential)]
struct zz { public z _z, __z, ___z; };
[StructLayout(LayoutKind.Sequential)]
struct zzz { private zz _zz; };
[StructLayout(LayoutKind.Sequential)]
struct zzzi { public zzz _zzz; int _i; };

/// ...

c = Marshal.SizeOf(typeof(z));      // 1
c = Marshal.SizeOf(typeof(zz));     // 3
c = Marshal.SizeOf(typeof(zzz));    // 3
c = Marshal.SizeOf(typeof(zzzi));   // 8

_ = IsZeroSizeStruct(typeof(z));    // true
_ = IsZeroSizeStruct(typeof(zz));   // true 
_ = IsZeroSizeStruct(typeof(zzz));  // true
_ = IsZeroSizeStruct(typeof(zzzi)); // false

[edit: see comment] What's strange here is that, when nesting 0-byte structs, the single-byte minimum can accumulate (i.e. into 3 bytes for 'zz' and 'zzz') but then suddenly all of that chaff disappears as soon as a single "substantial" field is included.

回答2:

It's the same reason zero-sized objects aren't allowed in C (or C++): pointer arithmetic in terms of number of elements.

C# supports pointer subtraction in unsafe blocks, defined thus:

Given two expressions, P and Q, of a pointer type T*, the expression P – Q computes the difference between the addresses given by P and Q and then divides that difference by sizeof(T).

Since division by zero is not possible, this implies that sizeof(T) > 0 for all T.

回答3:

Is this what you are looking for?

Null / Empty value for a struct in .Net 1.x

This solution mentions not having any overhead, which I believe is what you are looking for.

Furthermore, Stroustrup talks about why structs aren't empty in C++, now the language is different, but the principle is the same: http://www.stroustrup.com/bs_faq2.html#sizeof-empty

来源：https://stackoverflow.com/questions/16611598/why-does-an-empty-struct-in-c-sharp-consume-memory