C#与C++进行字段内存对齐

通过预留字段来补齐内存分配。在实际项目中采用这种方法较多，即保证了长度一致，也为以后的扩展提供了容错的可能性。

unsafe struct StructSequential
        {
            public fixed float x[8];
            public fixed float y[8];
            public fixed float z[8];
            public fixed float time[8];
            public fixed float radius[8];
            [MarshalAs(UnmanagedType.ByValArray, SizeConst = 8)]
            public float[] reserved;
        }

或者

public unsafe struct RTCPointQuery8
        {
            public fixed float x[8];
            public fixed float y[8];
            public fixed float z[8];
            public fixed float time[8];
            public fixed float radius[8];
        }
Span<byte> stack = stackalloc byte[sizeof(RTCPointQuery8) + 32];
ref var test = ref StackAllocAligned<RTCPointQuery8>(stack, 32);

static unsafe ref T StackAllocAligned<T>(Span<byte> stack, nuint alignment) where T : unmanaged
        {
            return ref Unsafe.AsRef<T>((void*)(((nint)Unsafe.AsPointer(ref MemoryMarshal.GetReference(stack))
                + ((nint)alignment - 1)) & ~(nint)(alignment - 1)));
        }

C#内存对齐有三种方式，分别是：LayoutKind.Sequential、LayoutKind.Explicit、LayoutKind.Auto。

LayoutKind.Sequential

顺序对齐，这是默认的内存对齐方式。会按照定义的struct中最大成员的长度进行内存对齐，使用这种对齐方式容易造成内存空间浪费。这种方式常用于和非托管代码交互的情形，CLR对struct的Layout处理方法与C/C++中默认的处理方式相同，都是按照结构体中占用空间最大的成员进行内存对齐。

[StructLayout(LayoutKind.Sequential)]
        struct StructSequential
        {
            public int A;
            public char B;
        }

        public static void Start()
        {
            StructSequential structSequential = new StructSequential();
            Debug.WriteLine("A的长度为：{0}", Marshal.SizeOf(structSequential.A));
            Debug.WriteLine("B的长度为：{0}", Marshal.SizeOf(structSequential.B));
            Debug.WriteLine("structSequential的长度为：{0}", Marshal.SizeOf(structSequential));
        }
//A的长度为：4
//B的长度为：1
//structSequential的长度为：8

如果我们正在创建一个与非托管代码没有任何交互的struct类型，我们也希望改变C#编译器的这种默认规则，因此，除了Sequential外，还有Explicit和Auto。

除此之外，你还可以通过设置StructLayout.Pack的值来达到内存大小分配一致，但这种方法可能会因硬件性能约束或者其他的问题。

[StructLayout(LayoutKind.Sequential, Pack = 32)]
        unsafe struct StructSequential
        {
            public fixed float x[8];
            public fixed float y[8];
            public fixed float z[8];
            public fixed float time[8];
            public fixed float radius[8];
        }

LayoutKind.Explicit

精确对齐，CLR会按照程序的设置对内存进行精确对齐。传入LayoutKind.Explicit，可以使字段按照我们设定的FieldOffset设置字段排序方式，CLR不对结构体进行内存对齐；但是，这种方式也是有缺陷的，如果设置错误，后果将比较严重。

[StructLayout(LayoutKind.Explicit)]
        struct StructSequential
        {
            [FieldOffset(0)]
            public int A;
            [FieldOffset(0)]
            public char B;
        }

        public static void Start()
        {
            StructSequential structSequential = new StructSequential();
            Debug.WriteLine("A的长度为：{0}", Marshal.SizeOf(structSequential.A));
            Debug.WriteLine("B的长度为：{0}", Marshal.SizeOf(structSequential.B));
            Debug.WriteLine("structSequential的长度为：{0}", Marshal.SizeOf(structSequential));
            structSequential.A = 0;
            structSequential.B = '0';
            Debug.WriteLine("structSequential.A: {0}", structSequential.A);
            Debug.WriteLine("structSequential.B: {0}", structSequential.B);
        }
//A的长度为：4
//B的长度为：1
//structSequential的长度为：4
//structSequential.A: 48
//structSequential.B: 0

[FieldOffset(0)]表示内存不会有任何偏移，这种情形下，同一字节中存储了两条数据，当其中一条改变时，另一条也随之而改变。

LayoutKind.Auto

自动对齐。CLR会调整struct中成员的顺序来自动对齐，使实例仅可能占用更少的内存，同时会进行一定字节的内存对齐。 按照自己选择的最优方式对实例中的字段进行排列。

[StructLayout(LayoutKind.Auto)]
        struct StructSequential
        {
            public int A;
            
            public char B;
        }

        public static void Start()
        {
            StructSequential structSequential = new StructSequential();
            Debug.WriteLine("A的长度为：{0}", Marshal.SizeOf(structSequential.A));
            Debug.WriteLine("B的长度为：{0}", Marshal.SizeOf(structSequential.B));
            //Debug.WriteLine("structSequential的长度为：{0}", Marshal.SizeOf(structSequential));
            unsafe
            {
                Debug.WriteLine("structSequential的长度为：{0}", sizeof(StructSequential));
            }
            structSequential.A = 0;
            structSequential.B = 'a';
            Debug.WriteLine("structSequential.A: {0}", structSequential.A);
            Debug.WriteLine("structSequential.B: {0}", structSequential.B);
        }