c# - 为什么结构 A 的大小与具有相同字段的结构 B 的大小不相等？

Question

为什么 struct A 的大小与 struct B 的大小不相等？

我需要做的是，它们的大小相同吗？

using System;

namespace ConsoleApplication1
{
    class Program
    {
        struct A
        {
            char a;
            char c;
            int b;
        }

        struct B
        {
            char a;
            int b;
            char c;

        }


        static void Main(string[] args)
        {
            unsafe
            {
                Console.WriteLine(sizeof(A));
                Console.WriteLine(sizeof(B));
            }
            Console.ReadLine();
        }
    }
}

输出是:

8
12

Answer 1

字段之间有一些填充。填充是使用前一个字段和下一个字段计算的。

另外，这个条件应该为真:

(size of struct) % (size of largest type) == 0

在您的例子中，最大的类型是 int，它的大小是 4 字节。

struct A
{
    char a; // size is 2, no previous field, next field size is 2 - no alignment needed
    char c; // size is 2, previous size is 2 -> 2 + 2 = 4, next size is 4 - no alignment needed
    int b;  //size is 4, it is last field, size is 4 + 4 = 8.  

    //current size is 2 + 2 + 4 = 8
    //8 % 4 == 0 - true - 8 is final size
}

struct B
{
    char a; // size is 2, next size is 4, alignment needed - 2 -> 4, size of this field with alignment is 4
    int b;  // size is 4, previous is 4, next size is 2(lower) - no alignment needed
    char c; // size is 2, previous is 4 + 4 = 8 - no alignment needed

    //current size is 4 + 4 + 2 = 10
    //but size should be size % 4 = 0 -> 10 % 4 == 0 - false, adjust to 12
}

如果您希望两个结构的大小相同，您可以使用 LayoutKind.Explicit:

[StructLayout(LayoutKind.Explicit)]
public struct A
{
    [FieldOffset(0)]
    char a;

    [FieldOffset(2)]
    char c;

    [FieldOffset(4)]
    int b;
}

[StructLayout(LayoutKind.Explicit)]
public struct B
{
    [FieldOffset(0)]
    char a;

    [FieldOffset(2)]
    int b;

    [FieldOffset(6)]
    char c;
}

或

您可以使用 LayoutKind.Sequential、Pack = 1 和 CharSet = CharSet.Unicode 来获得大小 8。

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Unicode)]
public struct A
{
    char a;
    char c;
    int b;
}

[StructLayout(LayoutKind.Sequential, Pack = 1, CharSet = CharSet.Unicode)]
public struct B
{        
    char a;
    int b;
    char c;
}

此外，您可以获得没有 unsafe 的结构大小:

Console.WriteLine(System.Runtime.InteropServices.Marshal.SizeOf(typeof(A)));
Console.WriteLine(System.Runtime.InteropServices.Marshal.SizeOf(typeof(B)));