如何在C#中封送（Marshal）动态大小的结构体数组？

为提升网络数据传输效率，想要将多个testStruct结构体打包到包含动态大小数组的sendTestStruct结构体中发送，而非分开发送单个结构体数据包，编写了如下代码：

public struct sendTestStruct
{
    public int numberOfStructs;
    public testStruct[] structs;
}
public struct testStruct 
{
    public float float1, float2;
    public int int1, int2;
    public byte byte1, byte2;
}

sendTestStruct send = new();
send.structs = new testStruct[5];
send.numberOfStructs = 5;
for (var i = 0; i < 5; i++)
{
    testStruct numbers = new();
    numbers.float1 = Random.Range(0f, 10f);
    numbers.float2 = Random.Range(0f, 10f);
    numbers.int1 = Random.Range(0, 10);
    numbers.int2 = Random.Range(0, 10);
    numbers.byte1 = (byte)Random.Range(0, 10);
    numbers.byte2 = (byte)Random.Range(0, 10);
    send.structs[i] = numbers;
}
IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(send));
Marshal.StructureToPtr(send, ptr, false);

sendTestStruct newstruct = Marshal.PtrToStructure<sendTestStruct>(ptr);

Marshal.FreeHGlobal(ptr);

执行sendTestStruct newstruct = Marshal.PtrToStructure<sendTestStruct>(ptr);时出现错误：

Structure field of type testStruct[] can't be marshalled as LPArray

方法一：添加Marshal特性并手动计算内存大小 #

问题根源是默认情况下，Marshal无法自动处理结构体中的动态数组，需要通过特性指定数组长度的来源，并手动计算总内存大小：

1. 修改结构体定义，添加布局特性和数组序列化特性：

[StructLayout(LayoutKind.Sequential)]
public struct sendTestStruct
{
    public int numberOfStructs;
    // SizeParamIndex = 0 表示数组长度由第0个字段（numberOfStructs）指定
    [MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)]
    public testStruct[] structs;
}

[StructLayout(LayoutKind.Sequential)]
public struct testStruct 
{
    public float float1, float2;
    public int int1, int2;
    public byte byte1, byte2;
}

2. 手动计算需要分配的内存总大小，替换原Marshal.SizeOf(send)的调用：

sendTestStruct send = new();
send.structs = new testStruct[5];
send.numberOfStructs = 5;
for (var i = 0; i < 5; i++)
{
    testStruct numbers = new();
    numbers.float1 = Random.Range(0f, 10f);
    numbers.float2 = Random.Range(0f, 10f);
    numbers.int1 = Random.Range(0, 10);
    numbers.int2 = Random.Range(0, 10);
    numbers.byte1 = (byte)Random.Range(0, 10);
    numbers.byte2 = (byte)Random.Range(0, 10);
    send.structs[i] = numbers;
}

// 手动计算总内存：结构体数量的大小 + 每个testStruct的大小 × 数量
int singleTestStructSize = Marshal.SizeOf<testStruct>();
int totalMemorySize = Marshal.SizeOf<int>() + send.numberOfStructs * singleTestStructSize;

IntPtr ptr = Marshal.AllocHGlobal(totalMemorySize);
Marshal.StructureToPtr(send, ptr, false);

sendTestStruct newstruct = Marshal.PtrToStructure<sendTestStruct>(ptr);

Marshal.FreeHGlobal(ptr);

方法二：手动序列化/反序列化为字节数组（更适合网络传输） #

如果是用于网络传输，直接手动将数据转换为字节数组会更灵活，无需依赖Marshal对结构体数组的处理：

序列化代码（转为可发送的字节数组）：

sendTestStruct send = new();
send.structs = new testStruct[5];
send.numberOfStructs = 5;
// 填充数据...

List<byte> byteBuffer = new List<byte>();
// 先写入结构体数量
byteBuffer.AddRange(BitConverter.GetBytes(send.numberOfStructs));

// 逐个序列化testStruct并加入缓冲区
int singleStructSize = Marshal.SizeOf<testStruct>();
foreach (var ts in send.structs)
{
    IntPtr tempPtr = Marshal.AllocHGlobal(singleStructSize);
    Marshal.StructureToPtr(ts, tempPtr, false);
    
    byte[] structBytes = new byte[singleStructSize];
    Marshal.Copy(tempPtr, structBytes, 0, singleStructSize);
    byteBuffer.AddRange(structBytes);
    
    Marshal.FreeHGlobal(tempPtr);
}

byte[] sendData = byteBuffer.ToArray();
// 此时sendData可直接通过网络发送

反序列化代码（从接收的字节数组恢复结构体）：

byte[] receivedData = ...; // 从网络接收的字节数组
sendTestStruct receivedStruct = new();

int offset = 0;
// 先读取结构体数量
receivedStruct.numberOfStructs = BitConverter.ToInt32(receivedData, offset);
offset += sizeof(int);

// 逐个恢复testStruct
int singleStructSize = Marshal.SizeOf<testStruct>();
receivedStruct.structs = new testStruct[receivedStruct.numberOfStructs];
for (int i = 0; i < receivedStruct.numberOfStructs; i++)
{
    IntPtr tempPtr = Marshal.AllocHGlobal(singleStructSize);
    Marshal.Copy(receivedData, offset, tempPtr, singleStructSize);
    
    receivedStruct.structs[i] = Marshal.PtrToStructure<testStruct>(tempPtr);
    
    Marshal.FreeHGlobal(tempPtr);
    offset += singleStructSize;
}

内容的提问来源于stack exchange，提问作者theepic0