C#使用struct直接轉換下位機資料

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, buffer, false);
    byte[] rawDatas = new byte[rawSize];
    Marshal.Copy(buffer, rawDatas, 0, 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, Size = 64, 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, SizeConst = 6, 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, Size = 64, 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,
            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;
    }

}

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#629120
• https://stackoverflow.com/questions/2871/reading-a-c-c-data-structure-in-c-sharp-from-a-byte-array/41836532#41836532