为什么C#中的空结构会占用内存 [英] Why does an empty struct in C# consume memory

问题描述

我一直都理解结构(值类型)包含的字节数与结构字段中定义的字节数完全相同...但是，我进行了一些测试，空结构似乎有例外:

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;
    }
}

如果运行该程序，您会发现FooStruct显然具有0字节的数据，它将占用1字节的内存.这对我来说是个问题，原因是我希望 Bar< FooStruct> 恰好消耗4个字节(因为我要分配很多).

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).

为什么会有这种行为，并且有一种解决方法(例如，是否有一种特殊的东西消耗了0个字节，我不打算进行重新设计)?

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)?

推荐答案

摘要 :. NET中的空结构占用1个字节.您可以将其视为 packing ，因为未命名的字节只能通过不安全的代码访问.

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.

更多信息:如果您根据.NET报告的值进行所有指针运算，则工作将始终如一.

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

下面的示例说明了在堆栈上使用相邻的0字节结构，但是这些观察结果显然也适用于0字节结构的数组.

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
}

正如我在评论中提到的那样，@ supercat在他指出时说得很对:从一开始就设计.NET以允许零长度结构可能没有任何问题，但是可能有些事情如果现在就开始这样做，那将会中断."

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."

编辑:如果您需要以编程方式区分0字节和1字节值类型，则可以使用以下代码:

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));
}

请注意，这可以正确标识总大小为零的任意嵌套结构.

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:查看评论] 奇怪的是，当嵌套0字节的结构时，单字节的最小值可以累积(即"zz"和"zzz"的3个字节)，但是突然之间所有只要包含一个实质性"字段，谷壳中的一部分就会消失.

[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.

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