C#使用struct直接转换下位机数据的示例代码

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/15020402
public 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

https://stackoverflow.com/questions/2871/reading-a-c-c-data-structure-in-c-sharp-from-a-byte-array/41836532

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