编写上位机与下位机通信的时候，涉及到协议的转换，比较多会使用到二进制。传统的方法，是将数据整体获取到byte数组中，然后逐字节对数据进行解析。这样操作工作量比较大，对于较长数据段更容易计算位置出错。
其实，对于下位机给出通讯的数据结构的情况下，可以直接使用C#的struct将数据直接转换。需要使用到Marshal。
数据结构
假定下位机（C语言编写）给到我们的数据结构是这个，传输方式为小端方式
typedef struct { unsigned long int time; // 4个字节 float tmpr[3]; // 4*3 个字节 float forces[6]; // 4*6个字节 float distance[6]; // 4*6个字节} dataItem_t;方法1
首先需要定义一个struct：
[StructLayout(LayoutKind.Sequential, Size = 64, Pack = 1)]public struct HardwareData{ //[FieldOffset(0)] public UInt32 Time; // 4个字节 [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3)] //[FieldOffset(4)] public float[] Tmpr; // 3* 4个字节 //[FieldOffset(16)] [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6)] public float[] Forces; // 6* 4个字节 //[FieldOffset(40)] [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6)] public float[] Distance; // 6*4个字节}然后使用以下代码进行转换
// code from https://stackoverflow.com/questions/628843/byte-for-byte-serialization-of-a-struct-in-c-sharp/629120#629120/// <summary>/// converts byte[] to struct/// </summary>public static T RawDeserialize<T>(byte[] rawData, int position){ int rawsize = Marshal.SizeOf(typeof(T)); if (rawsize > rawData.Length - position) throw new ArgumentException("Not enough data to fill struct. Array length from position: " + (rawData.Length - position) + ", Struct length: " + rawsize); IntPtr buffer = Marshal.AllocHGlobal(rawsize); Marshal.Copy(rawData, position, buffer, rawsize); T retobj = (T)Marshal.PtrToStructure(buffer, typeof(T)); Marshal.FreeHGlobal(buffer); return retobj;}/// <summary>/// converts a struct to byte[]/// </summary>public static byte[] RawSerialize(object anything){ int rawSize = Marshal.SizeOf(anything); IntPtr buffer = Marshal.AllocHGlobal(rawSize); Marshal.StructureToPtr(anything, buffer, false); byte[] rawDatas = new byte[rawSize]; Marshal.Copy(buffer, rawDatas, 0, rawSize); Marshal.FreeHGlobal(buffer); return rawDatas;}注意这里我使用的方式为LayoutKind.Sequential，如果直接使用LayoutKind.Explicit并设置FieldOffset会弹出一个诡异的错误System.TypeLoadException:"Could not load type 'ConsoleApp3.DataItem' from assembly 'ConsoleApp3, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' because it contains an object field at offset 4 that is incorrectly aligned or overlapped by a non-object field."。
方法2
提示是对齐的错误，这个和编译的时候使用的32bit和64位是相关的，详细数据封送对齐的操作我不就详细说了，贴下代码。
//强制指定x86编译[StructLayout(LayoutKind.Explicit, Size = 64, Pack = 1)]public struct DataItem{ [MarshalAs(UnmanagedType.U4)] [FieldOffset(0)] public UInt32 time; // 4个字节 [MarshalAs(UnmanagedType.ByValArray, SizeConst = 3, ArraySubType = UnmanagedType.R4)] [FieldOffset(4)] public float[] tmpr; // 3* 4个字节 [FieldOffset(16)] [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6, ArraySubType = UnmanagedType.R4)] public float[] forces; // 6* 4个字节 [FieldOffset(40)] [MarshalAs(UnmanagedType.ByValArray, SizeConst = 6, ArraySubType = UnmanagedType.R4)] public float[] distance; // 6*4个字节}强制指定x64编译没有成功，因为数据对齐后和从下位机上来的数据长度是不符的。
方法3
微软不是很推荐使用LayoutKind.Explicit，如果非要用并且不想指定平台的话，可以使用指针来操作，当然，这个需要unsafe。
var item = RawDeserialize<DataItem>(tail.ToArray(), 0);unsafe{ float* p = &item.forces; for (int i = 0; i < 6; i++) { Console.WriteLine(*p); p++; }}[StructLayout(LayoutKind.Explicit, Size = 64, Pack = 1)]public struct DataItem{ [FieldOffset(0)] public UInt32 time; // 4个字节 [FieldOffset(4)] public float tmpr; // 3* 4个字节 [FieldOffset(16)] public float forces; // 6* 4个字节 [FieldOffset(40)] public float distance; // 6*4个字节}方法4
感觉写起来还是很麻烦，既然用上了unsafe，就干脆直接一点。
[StructLayout(LayoutKind.Sequential, Pack = 1)]public unsafe struct DataItem{ public UInt32 time; // 4个字节 public fixed float tmpr[3]; // 3* 4个字节 public fixed float forces[6]; // 6* 4个字节 public fixed float distance[6]; // 6*4个字节}这样，获得数组可以直接正常访问，不再需要unsafe了。
总结
数据解析作为上下位机通讯的常用操作，使用struct直接转换数据可以大大简化工作量。建议还是使用LayoutKind.Sequential来进行封送数据，有关于数据在托管与非托管中的转换，可以详细看看微软有关互操作的内容。
以上代码在.NET 5.0下编译通过并能正常执行。
补充
注意上面的前提要求是字节序为小端字节序（一般计算机都是小端字节序），对于大端字节序发送过来的数据，需要进行字节序转换。我找到一处代码写的很好：
//CODE FROM https://stackoverflow.com/a/15020402public static class FooTest{ [StructLayout(LayoutKind.Sequential, Pack = 1)] public struct Foo2 { public byte b1; public short s; public ushort S; public int i; public uint I; public long l; public ulong L; public float f; public double d; [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)] public string MyString; } [StructLayout(LayoutKind.Sequential, Pack = 1)] public struct Foo { public byte b1; public short s; public ushort S; public int i; public uint I; public long l; public ulong L; public float f; public double d; [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 10)] public string MyString; public Foo2 foo2; } public static void test() { Foo2 sample2 = new Foo2() { b1 = 0x01, s = 0x0203, S = 0x0405, i = 0x06070809, I = 0x0a0b0c0d, l = 0xe0f101112131415, L = 0x161718191a1b1c, f = 1.234f, d = 4.56789, MyString = @"123456789", // null terminated => only 9 characters! }; Foo sample = new Foo() { b1 = 0x01, s = 0x0203, S = 0x0405, i = 0x06070809, I = 0x0a0b0c0d, l = 0xe0f101112131415, L = 0x161718191a1b1c, f = 1.234f, d = 4.56789, MyString = @"123456789", // null terminated => only 9 characters! foo2 = sample2, }; var bytes_LE = Dummy.StructToBytes(sample, Endianness.LittleEndian); var restoredLEAsLE = Dummy.BytesToStruct<Foo>(bytes_LE, Endianness.LittleEndian); var restoredLEAsBE = Dummy.BytesToStruct<Foo>(bytes_LE, Endianness.BigEndian); var bytes_BE = Dummy.StructToBytes(sample, Endianness.BigEndian); var restoredBEAsLE = Dummy.BytesToStruct<Foo>(bytes_BE, Endianness.LittleEndian); var restoredBEAsBE = Dummy.BytesToStruct<Foo>(bytes_BE, Endianness.BigEndian); Debug.Assert(sample.Equals(restoredLEAsLE)); Debug.Assert(sample.Equals(restoredBEAsBE)); Debug.Assert(restoredBEAsLE.Equals(restoredLEAsBE)); } public enum Endianness { BigEndian, LittleEndian } private static void MaybeAdjustEndianness(Type type, byte[] data, Endianness endianness, int startOffset = 0) { if ((BitConverter.IsLittleEndian) == (endianness == Endianness.LittleEndian)) { // nothing to change => return return; } foreach (var field in type.GetFields()) { var fieldType = field.FieldType; if (field.IsStatic) // don't process static fields continue; if (fieldType == typeof(string)) // don't swap bytes for strings continue; var offset = Marshal.OffsetOf(type, field.Name).ToInt32(); // handle enums if (fieldType.IsEnum) fieldType = Enum.GetUnderlyingType(fieldType); // check for sub-fields to recurse if necessary var subFields = fieldType.GetFields().Where(subField => subField.IsStatic == false).ToArray(); var effectiveOffset = startOffset + offset; if (subFields.Length == 0) { Array.Reverse(data, effectiveOffset, Marshal.SizeOf(fieldType)); } else { // recurse MaybeAdjustEndianness(fieldType, data, endianness, effectiveOffset); } } } internal static T BytesToStruct<T>(byte[] rawData, Endianness endianness) where T : struct { T result = default(T); MaybeAdjustEndianness(typeof(T), rawData, endianness); GCHandle handle = GCHandle.Alloc(rawData, GCHandleType.Pinned); try { IntPtr rawDataPtr = handle.AddrOfPinnedObject(); result = (T)Marshal.PtrToStructure(rawDataPtr, typeof(T)); } finally { handle.Free(); } return result; } internal static byte[] StructToBytes<T>(T data, Endianness endianness) where T : struct { byte[] rawData = new byte[Marshal.SizeOf(data)]; GCHandle handle = GCHandle.Alloc(rawData, GCHandleType.Pinned); try { IntPtr rawDataPtr = handle.AddrOfPinnedObject(); Marshal.StructureToPtr(data, rawDataPtr, false); } finally { handle.Free(); } MaybeAdjustEndianness(typeof(T), rawData, endianness); return rawData; }}参考资料
- https://www.developerfusion.com/article/84519/mastering-structs-in-c/
- https://stackoverflow.com/a/15020402
- https://stackoverflow.com/questions/628843/byte-for-byte-serialization-of-a-struct-in-c-sharp/629120#629120
- https://stackoverflow.com/questions/2871/reading-a-c-c-data-structure-in-c-sharp-from-a-byte-array/41836532#41836532
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编写上位机与下位机通信的时候，涉及到协议的转换，比较多会使用到二进制。传统的方法，是将数据整体获取到byte数组中，然后逐字节对数据进行解析。这样操作工作量比较大，对于较长数据段更容易计算位置出错。其实，对于下位机给出通讯的数据结构的情况下，可以直接使用C#的struct将数据直接转换。需要使用到Marshal。数据结构假定下位机（C语言编写）给到我们的数据结构是这个，传输方式为小端方式typed
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