C＃中的位字段

80

我有一个需要填充并写入磁盘的结构（实际上有几个）。

一个例子是：

byte-6    
bit0 - original_or_copy  
bit1 - copyright  
bit2 - data_alignment_indicator  
bit3 - PES_priority  
bit4-bit5 - PES_scrambling control.  
bit6-bit7 - reserved

在CI中可能会执行以下操作：

struct PESHeader  {
    unsigned reserved:2;
    unsigned scrambling_control:2;
    unsigned priority:1;
    unsigned data_alignment_indicator:1;
    unsigned copyright:1;
    unsigned original_or_copy:1;
};

有什么办法可以在C＃中做到这一点，从而使我能够使用结构取消引用点运算符来访问这些位？

对于两个结构，我只需要包装访问器函数中的位移即可。

我有大量的结构以这种方式处理，因此我正在寻找更易于阅读和编写的东西。

c#  bit-fields 
— 抢
source

Answers:

56

我可能会使用属性将某些东西组合在一起，然后使用一个转换类将适当地归因于结构的结构转换为位域基元。就像是...

using System;

namespace BitfieldTest
{
    [global::System.AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
    sealed class BitfieldLengthAttribute : Attribute
    {
        uint length;

        public BitfieldLengthAttribute(uint length)
        {
            this.length = length;
        }

        public uint Length { get { return length; } }
    }

    static class PrimitiveConversion
    {
        public static long ToLong<T>(T t) where T : struct
        {
            long r = 0;
            int offset = 0;

            // For every field suitably attributed with a BitfieldLength
            foreach (System.Reflection.FieldInfo f in t.GetType().GetFields())
            {
                object[] attrs = f.GetCustomAttributes(typeof(BitfieldLengthAttribute), false);
                if (attrs.Length == 1)
                {
                    uint fieldLength  = ((BitfieldLengthAttribute)attrs[0]).Length;

                    // Calculate a bitmask of the desired length
                    long mask = 0;
                    for (int i = 0; i < fieldLength; i++)
                        mask |= 1 << i;

                    r |= ((UInt32)f.GetValue(t) & mask) << offset;

                    offset += (int)fieldLength;
                }
            }

            return r;
        }
    }

    struct PESHeader
    {
        [BitfieldLength(2)]
        public uint reserved;
        [BitfieldLength(2)]
        public uint scrambling_control;
        [BitfieldLength(1)]
        public uint priority;
        [BitfieldLength(1)]
        public uint data_alignment_indicator;
        [BitfieldLength(1)]
        public uint copyright;
        [BitfieldLength(1)]
        public uint original_or_copy;
    };

    public class MainClass
    {
        public static void Main(string[] args)
        {
            PESHeader p = new PESHeader();

            p.reserved = 3;
            p.scrambling_control = 2;
            p.data_alignment_indicator = 1;

            long l = PrimitiveConversion.ToLong(p);


            for (int i = 63; i >= 0; i--)
            {
                Console.Write( ((l & (1l << i)) > 0) ? "1" : "0");
            }

            Console.WriteLine();

            return;
        }
    }
}

产生预期的... 000101011。当然，它需要更多的错误检查和更精巧的键入，但是（我认为）该概念是合理的，可重用的，并且可以让您轻松打出一打维护的结构。

亚当

— 亚当·赖特
source
9
注意：根据MSDN，“该GetFields方法不会以特定顺序（例如，字母顺序或声明顺序）返回字段。您的代码不得依赖于字段的返回顺序，因为该顺序会有所不同。” 这不是在这里引起问题吗？
— 凯文·P·赖斯
1
如果创建IBitfield“标记”接口（没有成员），则可以将PrimitiveConversion类转换为任何实现的结构的扩展方法IBitfield。例如：public static long ToLong(this IBitfield obj) {}。然后，该ToLong()方法将出现在IntellisenseIBitfield对象中。
— 凯文·莱斯
您可以使用'f.SetValue（t，someValue）'撤消该过程吗？我正在使用它来将数据包类转换为用于套接字传输的消息缓冲区。效果很好，但是由于某些原因，我无法使用f.SetValue（）将数据从流读取回结构。没有错误，就是行不通。
— buzzard51 '16
GetFields可能会由于反射缓存而遭受重新排序的问题，但是按“ MetadataToken”进行排序应该可以解决该问题（您可以通过混合获取单个字段并以某种顺序获取所有字段来进行重现）。
— firda
26

通过使用枚举，您可以执行此操作，但是看起来很尴尬。

[Flags]
public enum PESHeaderFlags
{
    IsCopy = 1, // implied that if not present, then it is an original
    IsCopyrighted = 2,
    IsDataAligned = 4,
    Priority = 8,
    ScramblingControlType1 = 0,
    ScramblingControlType2 = 16,
    ScramblingControlType3 = 32,
    ScramblingControlType4 = 16+32,
    ScramblingControlFlags = ScramblingControlType1 | ScramblingControlType2 | ... ype4
    etc.
}
— 拉瑟诉卡尔森
source
20

您想要StructLayoutAttribute

[StructLayout(LayoutKind.Explicit, Size=1, CharSet=CharSet.Ansi)]
public struct Foo 
{ [FieldOffset(0)]public byte original_or_copy; 
  [FieldOffset(0)]public byte copyright;
  [FieldOffset(0)]public byte data_alignment_indicator; 
  [FieldOffset(0)]public byte PES_priority; 
  [FieldOffset(0)]public byte PES_scrambling_control; 
  [FieldOffset(0)]public byte reserved; 
}

这实际上是一个并集，但您可以将其用作位域-您只需要知道每个字段的位应该在字节中的哪个位置即可。实用程序函数和/或与AND相对的常量会有所帮助。

const byte _original_or_copy = 1;
const byte _copyright        = 2;

//bool ooo = foo.original_or_copy();
static bool original_or_copy(this Foo foo) 
{ return  (foo.original_or_copy & _original_or_copy)  == original_or_copy;
}

还有LayoutKind.Sequential，它可以让您以C方式进行操作。

— 马克·西达德
source
17

正如Christophe Lambrechts所建议的那样，BitVector32提供了一种解决方案。抖动性能应该足够，但是不确定。这是说明此解决方案的代码：

public struct rcSpan
{
    //C# Spec 10.4.5.1: The static field variable initializers of a class correspond to a sequence of assignments that are executed in the textual order in which they appear in the class declaration.
    internal static readonly BitVector32.Section sminSection = BitVector32.CreateSection(0x1FFF);
    internal static readonly BitVector32.Section smaxSection = BitVector32.CreateSection(0x1FFF, sminSection);
    internal static readonly BitVector32.Section areaSection = BitVector32.CreateSection(0x3F, smaxSection);

    internal BitVector32 data;

    //public uint smin : 13; 
    public uint smin
    {
        get { return (uint)data[sminSection]; }
        set { data[sminSection] = (int)value; }
    }

    //public uint smax : 13; 
    public uint smax
    {
        get { return (uint)data[smaxSection]; }
        set { data[smaxSection] = (int)value; }
    }

    //public uint area : 6; 
    public uint area
    {
        get { return (uint)data[areaSection]; }
        set { data[areaSection] = (int)value; }
    }
}

您可以通过这种方式做很多事情。通过为每个字段提供手工访问器，您甚至可以在不使用BitVector32的情况下做得更好：

public struct rcSpan2
{
    internal uint data;

    //public uint smin : 13; 
    public uint smin
    {
        get { return data & 0x1FFF; }
        set { data = (data & ~0x1FFFu ) | (value & 0x1FFF); }
    }

    //public uint smax : 13; 
    public uint smax
    {
        get { return (data >> 13) & 0x1FFF; }
        set { data = (data & ~(0x1FFFu << 13)) | (value & 0x1FFF) << 13; }
    }

    //public uint area : 6; 
    public uint area
    {
        get { return (data >> 26) & 0x3F; }
        set { data = (data & ~(0x3F << 26)) | (value & 0x3F) << 26; }
    }
}

令人惊讶的是，这最后一个手工解决方案似乎是最方便，最不费力且最短的解决方案。当然，那只是我个人的喜好。

— Zbyl
source
8

根据Zbyl的回答，又有一个。这对我来说更容易更改-我只需要调整sz0，sz1 ...并确保Set / Get块中的mask＃和loc＃是正确的。

在性能方面，应该与它们都解析为38个MSIL语句相同。（常量在编译时解析）

public struct MyStruct
{
    internal uint raw;

    const int sz0 = 4, loc0 = 0,          mask0 = ((1 << sz0) - 1) << loc0;
    const int sz1 = 4, loc1 = loc0 + sz0, mask1 = ((1 << sz1) - 1) << loc1;
    const int sz2 = 4, loc2 = loc1 + sz1, mask2 = ((1 << sz2) - 1) << loc2;
    const int sz3 = 4, loc3 = loc2 + sz2, mask3 = ((1 << sz3) - 1) << loc3;

    public uint Item0
    {
        get { return (uint)(raw & mask0) >> loc0; }
        set { raw = (uint)(raw & ~mask0 | (value << loc0) & mask0); }
    }

    public uint Item1
    {
        get { return (uint)(raw & mask1) >> loc1; }
        set { raw = (uint)(raw & ~mask1 | (value << loc1) & mask1); }
    }

    public uint Item2
    {
        get { return (uint)(raw & mask2) >> loc2; }
        set { raw = (uint)(raw & ~mask2 | (value << loc2) & mask2); }
    }

    public uint Item3
    {
        get { return (uint)((raw & mask3) >> loc3); }
        set { raw = (uint)(raw & ~mask3 | (value << loc3) & mask3); }
    }
}
— 日落任务
source
1
很棒的设置。重用喜悦;）。我发现，当位域为“满”时（例如，当设置时raw=uint.MaxValue），我确实必须稍微更改最后一个项目。或者，也许只考虑最后一个属性。不确定。因此，对于上面的示例，ItemX属性获取器如下所示：get { return (uint)((Raw & Mask3) >> Loc3); }. The setter look like this: set {Raw =（uint）（Raw＆〜Mask3 |（value << Loc3）＆Mask3）; }`如果不进行此更改，则最后一个属性的转换将失败。
— Spiralis
1
@Spiralis：感谢您的注意。我按照您说的进行了更新，现在效果更好。
— Sunsetquest 2013年
5

虽然这是一门课，但使用BitArray似乎是最不容易发明车轮的方法。除非您对性能有真正的要求，否则这是最简单的选择。（索引可以由[]运算符引用。）

— 康拉德
source
3

我认为，如果将标志枚举设置为字节枚举，它也可以工作：

[Flags] enum PesHeaders : byte { /* ... */ }
— 马克·西达德
source
2

我写了一个，分享一下，可能会帮助某人：

[global::System.AttributeUsage(AttributeTargets.Field, AllowMultiple = false)]
public sealed class BitInfoAttribute : Attribute {
    byte length;
    public BitInfoAttribute(byte length) {
        this.length = length;
    }
    public byte Length { get { return length; } }
}

public abstract class BitField {

    public void parse<T>(T[] vals) {
        analysis().parse(this, ArrayConverter.convert<T, uint>(vals));
    }

    public byte[] toArray() {
        return ArrayConverter.convert<uint, byte>(analysis().toArray(this));
    }

    public T[] toArray<T>() {
        return ArrayConverter.convert<uint, T>(analysis().toArray(this));
    }

    static Dictionary<Type, BitTypeInfo> bitInfoMap = new Dictionary<Type, BitTypeInfo>();
    private BitTypeInfo analysis() {
        Type type = this.GetType();
        if (!bitInfoMap.ContainsKey(type)) {
            List<BitInfo> infos = new List<BitInfo>();

            byte dataIdx = 0, offset = 0;
            foreach (System.Reflection.FieldInfo f in type.GetFields()) {
                object[] attrs = f.GetCustomAttributes(typeof(BitInfoAttribute), false);
                if (attrs.Length == 1) {
                    byte bitLen = ((BitInfoAttribute)attrs[0]).Length;
                    if (offset + bitLen > 32) {
                        dataIdx++;
                        offset = 0;
                    }
                    infos.Add(new BitInfo(f, bitLen, dataIdx, offset));
                    offset += bitLen;
                }
            }
            bitInfoMap.Add(type, new BitTypeInfo(dataIdx + 1, infos.ToArray()));
        }
        return bitInfoMap[type];
    }
}

class BitTypeInfo {
    public int dataLen { get; private set; }
    public BitInfo[] bitInfos { get; private set; }

    public BitTypeInfo(int _dataLen, BitInfo[] _bitInfos) {
        dataLen = _dataLen;
        bitInfos = _bitInfos;
    }

    public uint[] toArray<T>(T obj) {
        uint[] datas = new uint[dataLen];
        foreach (BitInfo bif in bitInfos) {
            bif.encode(obj, datas);
        }
        return datas;
    }

    public void parse<T>(T obj, uint[] vals) {
        foreach (BitInfo bif in bitInfos) {
            bif.decode(obj, vals);
        }
    }
}

class BitInfo {

    private System.Reflection.FieldInfo field;
    private uint mask;
    private byte idx, offset, shiftA, shiftB;
    private bool isUnsigned = false;

    public BitInfo(System.Reflection.FieldInfo _field, byte _bitLen, byte _idx, byte _offset) {
        field = _field;
        mask = (uint)(((1 << _bitLen) - 1) << _offset);
        idx = _idx;
        offset = _offset;
        shiftA = (byte)(32 - _offset - _bitLen);
        shiftB = (byte)(32 - _bitLen);

        if (_field.FieldType == typeof(bool)
            || _field.FieldType == typeof(byte)
            || _field.FieldType == typeof(char)
            || _field.FieldType == typeof(uint)
            || _field.FieldType == typeof(ulong)
            || _field.FieldType == typeof(ushort)) {
            isUnsigned = true;
        }
    }

    public void encode(Object obj, uint[] datas) {
        if (isUnsigned) {
            uint val = (uint)Convert.ChangeType(field.GetValue(obj), typeof(uint));
            datas[idx] |= ((uint)(val << offset) & mask);
        } else {
            int val = (int)Convert.ChangeType(field.GetValue(obj), typeof(int));
            datas[idx] |= ((uint)(val << offset) & mask);
        }
    }

    public void decode(Object obj, uint[] datas) {
        if (isUnsigned) {
            field.SetValue(obj, Convert.ChangeType((((uint)(datas[idx] & mask)) << shiftA) >> shiftB, field.FieldType));
        } else {
            field.SetValue(obj, Convert.ChangeType((((int)(datas[idx] & mask)) << shiftA) >> shiftB, field.FieldType));
        }
    }
}

public class ArrayConverter {
    public static T[] convert<T>(uint[] val) {
        return convert<uint, T>(val);
    }

    public static T1[] convert<T0, T1>(T0[] val) {
        T1[] rt = null;
        // type is same or length is same
        // refer to http://stackoverflow.com/questions/25759878/convert-byte-to-sbyte
        if (typeof(T0) == typeof(T1)) { 
            rt = (T1[])(Array)val;
        } else {
            int len = Buffer.ByteLength(val);
            int w = typeWidth<T1>();
            if (w == 1) { // bool
                rt = new T1[len * 8];
            } else if (w == 8) {
                rt = new T1[len];
            } else { // w > 8
                int nn = w / 8;
                int len2 = (len / nn) + ((len % nn) > 0 ? 1 : 0);
                rt = new T1[len2];
            }

            Buffer.BlockCopy(val, 0, rt, 0, len);
        }
        return rt;
    }

    public static string toBinary<T>(T[] vals) {
        StringBuilder sb = new StringBuilder();
        int width = typeWidth<T>();
        int len = Buffer.ByteLength(vals);
        for (int i = len-1; i >=0; i--) {
            sb.Append(Convert.ToString(Buffer.GetByte(vals, i), 2).PadLeft(8, '0')).Append(" ");
        }
        return sb.ToString();
    }

    private static int typeWidth<T>() {
        int rt = 0;
        if (typeof(T) == typeof(bool)) { // x
            rt = 1;
        } else if (typeof(T) == typeof(byte)) { // x
            rt = 8;
        } else if (typeof(T) == typeof(sbyte)) {
            rt = 8;
        } else if (typeof(T) == typeof(ushort)) { // x
            rt = 16;
        } else if (typeof(T) == typeof(short)) {
            rt = 16;
        } else if (typeof(T) == typeof(char)) {
            rt = 16;
        } else if (typeof(T) == typeof(uint)) { // x
            rt = 32;
        } else if (typeof(T) == typeof(int)) {
            rt = 32;
        } else if (typeof(T) == typeof(float)) {
            rt = 32;
        } else if (typeof(T) == typeof(ulong)) { // x
            rt = 64;
        } else if (typeof(T) == typeof(long)) {
            rt = 64;
        } else if (typeof(T) == typeof(double)) {
            rt = 64;
        } else {
            throw new Exception("Unsupport type : " + typeof(T).Name);
        }
        return rt;
    }
}

和用法： 

class MyTest01 : BitField {
    [BitInfo(3)]
    public bool d0;
    [BitInfo(3)]
    public short d1;
    [BitInfo(3)]
    public int d2;
    [BitInfo(3)]
    public int d3;
    [BitInfo(3)]
    public int d4;
    [BitInfo(3)]
    public int d5;

    public MyTest01(bool _d0, short _d1, int _d2, int _d3, int _d4, int _d5) {
        d0 = _d0;
        d1 = _d1;
        d2 = _d2;
        d3 = _d3;
        d4 = _d4;
        d5 = _d5;
    }

    public MyTest01(byte[] datas) {
        parse(datas);
    }

    public new string ToString() {
        return string.Format("d0: {0}, d1: {1}, d2: {2}, d3: {3}, d4: {4}, d5: {5} \r\nbinary => {6}",
            d0, d1, d2, d3, d4, d5, ArrayConverter.toBinary(toArray()));
    }
};

class MyTest02 : BitField {
    [BitInfo(5)]
    public bool val0;
    [BitInfo(5)]
    public byte val1;
    [BitInfo(15)]
    public uint val2;
    [BitInfo(15)]
    public float val3;
    [BitInfo(15)]
    public int val4;
    [BitInfo(15)]
    public int val5;
    [BitInfo(15)]
    public int val6;

    public MyTest02(bool v0, byte v1, uint v2, float v3, int v4, int v5, int v6) {
        val0 = v0;
        val1 = v1;
        val2 = v2;
        val3 = v3;
        val4 = v4;
        val5 = v5;
        val6 = v6;
    }

    public MyTest02(byte[] datas) {
        parse(datas);
    }

    public new string ToString() {
        return string.Format("val0: {0}, val1: {1}, val2: {2}, val3: {3}, val4: {4}, val5: {5}, val6: {6}\r\nbinary => {7}",
            val0, val1, val2, val3, val4, val5, val6, ArrayConverter.toBinary(toArray()));
    }
}

public class MainClass {

    public static void Main(string[] args) {
        MyTest01 p = new MyTest01(false, 1, 2, 3, -1, -2);
        Debug.Log("P:: " + p.ToString());
        MyTest01 p2 = new MyTest01(p.toArray());
        Debug.Log("P2:: " + p2.ToString());

        MyTest02 t = new MyTest02(true, 1, 12, -1.3f, 4, -5, 100);
        Debug.Log("t:: " + t.ToString());
        MyTest02 t2 = new MyTest02(t.toArray());
        Debug.Log("t:: " + t.ToString());

        Console.Read();
        return;
    }
}
— 斯塔伦德
source
2

我发现自己对这些辅助功能很满意：

uint SetBits(uint word, uint value, int pos, int size)
{
    uint mask = ((((uint)1) << size) - 1) << pos;
    word &= ~mask; //resettiamo le posizioni
    word |= (value << pos) & mask;
    return word;
}

uint ReadBits(uint word, int pos, int size)
{
    uint mask = ((((uint)1) << size) - 1) << pos;
    return (word & mask) >> pos;
}

然后：

uint the_word;

public uint Itemx
{
    get { return ReadBits(the_word, 5, 2); }
    set { the_word = SetBits(the_word, value, 5, 2) }
}
— 维托·马洛达（Vito Marolda）
source
简单方便。这就是我所需要的。谢谢:)
— Prihex
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