转自：http://blog.chinaunix.net/uid-215617-id-2213082.html

Some of the C# code I've been writing recently communicates via TCP/IP with legacy C++ applications. These applications use a raw packet format where C/C++ structures are passed back and forth.

Here is a simplified example of what the legacy code could look like:

#pragma pack(1)
typedef struct
{
    int id;
    char[50] text;
} MESSAGE;

// Send a message
MESSAGE msg;
msg.id = 1;
strcpy(msg.text, "This is a test");
send(socket, (char*)&msg);

// Receive a message
char buffer[100];
recv(socket, buffer, 100);
MESSAGE* msg = (MESSAGE*)buffer;
printf("id=%d\n", msg->id);
printf("text=%s\n", msg->text);

The problem I was faced with was how to receive and handle this kind of message in a C# application. One method is to use BitConverter and Encoding.ASCII to grab the data field by field. This is tedious, prone to errors and easy to break of modifications are made in the future.

A better method is to marshal the byte array to a C# structure. Here is an example of how to do that marshaling:

using System;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, Pack=1)]
struct Message
{
    public int id;
    [MarshalAs (UnmanagedType.ByValTStr, SizeConst=50)] 
    public string text;
}

void OnPacket(byte[] packet)
{
    GCHandle pinnedPacket = GCHandle.Alloc(packet, GCHandleType.Pinned);
    Message msg = (Message)Marshal.PtrToStructure(
        pinnedPacket.AddrOfPinnedObject(),
        typeof(Message));        
    pinnedPacket.Free();
}

The GCHandle.Alloc call pins the byte[] in memory so the garbage collector doesn't mess with it. The AddrOfPinnedObject call returns an IntPtr pointing to the start of the array and the Marshal.PtrToStructure does the work of marshaling the byte[] to the structure.

If the actual structure data didn't start at the beginning of the byte array you would use the following assuming the structure data starts at position 10 of the array:

Message p = (Message)Marshal.PtrToStructure( Marshal.UnsafeAddrOfPinnedArrayElement(pinnedPacket, 10), typeof(Message));