C#使用struct直接转换下位机数据

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