編寫上位機與下位機通訊的時候，涉及到協議的轉換，比較多會使用到二進位制。傳統的方法，是將資料整體獲取到byte陣列中，然後逐位元組對資料進行解析。這樣操作工作量比較大，對於較長資料段更容易計算位置出錯。

其實，對於下位機給出通訊的資料結構的情況下，可以直接使用C#的struct將資料直接轉換。需要使用到Marshal。

資料結構

假定下位機（C語言編寫）給到我們的資料結構是這個，傳輸方式為小端方式

typedef struct {
	unsigned long int time;   // 4個位元組
	float tmpr[3];     // 4*3 個位元組
	float forces[6];     // 4*6個位元組
	float distance[6];    // 4*6個位元組
} dataItem_t;

方法1

首先需要定義一個struct：

[StructLayout(LayoutKind.Sequential,Size = 64,Pack = 1)]
public struct HardwareData
{
 //[FieldOffset(0)]
 public UInt32 Time;   // 4個位元組
 [MarshalAs(UnmanagedType.ByValArray,SizeConst = 3)]
 //[FieldOffset(4)]
 public float[] Tmpr;     // 3* 4個位元組
 //[FieldOffset(16)]
 [MarshalAs(UnmanagedType.ByValArray,SizeConst = 6)]
 public float[] Forces;     // 6* 4個位元組
 //[FieldOffset(40)]
 [MarshalAs(UnmanagedType.ByValArray,SizeConst = 6)]
 public float[] Distance;    // 6*4個位元組
}

然後使用以下程式碼進行轉換

// 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,false);
 byte[] rawDatas = new byte[rawSize];
 Marshal.Copy(buffer,rawDatas,rawSize);
 Marshal.FreeHGlobal(buffer);
 return rawDatas;
}

注意這裡我使用的方式為LayoutKind.Sequential，如果直接使用LayoutKind.Explicit並設定FieldOffset會彈出一個詭異的錯誤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

提示是對齊的錯誤，這個和編譯的時候使用的32bit和64位是相關的，詳細資料封送對齊的操作我不就詳細說了，貼下程式碼。

//強制指定x86編譯
[StructLayout(LayoutKind.Explicit,Pack = 1)]
public struct DataItem
{
 [MarshalAs(UnmanagedType.U4)]
 [FieldOffset(0)]
 public UInt32 time;   // 4個位元組
 [MarshalAs(UnmanagedType.ByValArray,SizeConst = 3,ArraySubType = UnmanagedType.R4)]
 [FieldOffset(4)]
 public float[] tmpr;     // 3* 4個位元組
 [FieldOffset(16)]
 [MarshalAs(UnmanagedType.ByValArray,SizeConst = 6,ArraySubType = UnmanagedType.R4)]
 public float[] forces;     // 6* 4個位元組
 [FieldOffset(40)]
 [MarshalAs(UnmanagedType.ByValArray,ArraySubType = UnmanagedType.R4)]
 public float[] distance;    // 6*4個位元組
}

強制指定x64編譯沒有成功，因為資料對齊後和從下位機上來的資料長度是不符的。

方法3

微軟不是很推薦使用LayoutKind.Explicit，如果非要用並且不想指定平臺的話，可以使用指標來操作，當然，這個需要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,Pack = 1)]
public struct DataItem
{
 [FieldOffset(0)]
 public UInt32 time;   // 4個位元組
 [FieldOffset(4)]
 public float tmpr;     // 3* 4個位元組
 [FieldOffset(16)]
 public float forces;     // 6* 4個位元組
 [FieldOffset(40)]
 public float distance;    // 6*4個位元組
}

方法4

感覺寫起來還是很麻煩，既然用上了unsafe，就乾脆直接一點。

[StructLayout(LayoutKind.Sequential,Pack = 1)]
public unsafe struct DataItem
{
 public UInt32 time;   // 4個位元組
 public fixed float tmpr[3];     // 3* 4個位元組
 public fixed float forces[6];     // 6* 4個位元組
 public fixed float distance[6];    // 6*4個位元組
}

這樣，獲得陣列可以直接正常訪問，不再需要unsafe了。

總結

資料解析作為上下位機通訊的常用操作，使用struct直接轉換資料可以大大簡化工作量。建議還是使用LayoutKind.Sequential來進行封送資料，有關於資料在託管與非託管中的轉換，可以詳細看看微軟有關互操作的內容。

以上程式碼在.NET 5.0下編譯通過並能正常執行。

補充

注意上面的前提要求是位元組序為小端位元組序（一般計算機都是小端位元組序），對於大端位元組序傳送過來的資料，需要進行位元組序轉換。我找到一處程式碼寫的很好：

//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,// 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),endianness);

  return rawData;
 }

}

參考資料

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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