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问题:
I have a structure which I need to populate and write to disk (several actually).
An example is:
byte-6 bit0 - original_or_copy bit1 - copyright bit2 - data_alignment_indicator bit3 - PES_priority bit4-bit5 - PES_scrambling control. bit6-bit7 - reserved
In C I might do something like the following:
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; };
Is there any way to do this in C# that would enable me to access the bits using the struct dereferencing dot operator?
For a couple of structures, I can just do bit shifting wrapped in an accessor function.
I have loads of structures to handle in this way, so I'm looking for something that's easier to read and quicker to write.
回答1:
I'd probably knock together something using attributes, then a conversion class to convert suitably attributed structures to the bitfield primitives. Something like...
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) 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 = 0; i--) { Console.Write( ((l & (1l 0) ? "1" : "0"); } Console.WriteLine(); return; } } }
Which produces the expected ...000101011. Of course, it needs more error checking and a slightly saner typing, but the concept is (I think) sound, reusable, and lets you knock out easily maintained structures by the dozen.
adamw
回答2:
By using an enum you can do this, but will look awkward.
[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. }
回答3:
You want 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; }
This is really a union but you can use it as a bitfield--you just have to be conscious of where in the byte the bits for each field are supposed to be. Utility functions and/or constants to AND against can help.
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; }
There is also LayoutKind.Sequential which will allow you to do it the C way.
回答4:
As Christophe Lambrechts suggested BitVector32 provides a solution. Jitted performance should be adequate, but don't know for sure. Here's the code illustrating this solution:
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; } } }
You can do a lot this way. You can do even better without using BitVector32, by providing handmade accessors for every field:
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 > 26) & 0x3F; } set { data = (data & ~(0x3F
Surprisingly this last, handmade solution seems to be the most convenient, least convoluted, and the shortest one. That's of course only my personal preference.
回答5:
You could also use the BitVector32 and especially the Section struct. The example is very good.
回答6:
One more based off of Zbyl's answer. This one is a little easier to change around for me - I just have to adjust the sz0,sz1... and also make sure mask# and loc# are correct in the Set/Get blocks.
Performance wise, it should be the same as they both resolved to 38 MSIL statements. (constants are resolved at compile time)
public struct MyStruct { internal uint raw; const int sz0 = 4, loc0 = 0, mask0 = ((1 > loc0; } set { raw = (uint)(raw & ~mask0 | (value > loc1; } set { raw = (uint)(raw & ~mask1 | (value > loc2; } set { raw = (uint)(raw & ~mask2 | (value > loc3); } set { raw = (uint)(raw & ~mask3 | (value
回答7:
While it is a class, using BitArray seems like the way to least reinvent the wheel. Unless you're really pressed for performance, this is the simplest option. (Indexes can be referenced with the [] operator.)
回答8:
A flags enum can work too, I think, if you make it a byte enum:
[Flags] enum PesHeaders : byte { /* ... */ }
回答9:
Could an Enum with the Flags Attribute help maybe? See here:
What does the [Flags] Enum Attribute mean in C#?
回答10:
I wrote one, share it, may help someone:
[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[] vals) { analysis().parse(this, ArrayConverter.convert(vals)); } public byte[] toArray() { return ArrayConverter.convert(analysis().toArray(this)); } public T[] toArray() { return ArrayConverter.convert(analysis().toArray(this)); } static Dictionary bitInfoMap = new Dictionary(); private BitTypeInfo analysis() { Type type = this.GetType(); if (!bitInfoMap.ContainsKey(type)) { List infos = new List(); 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 obj) { uint[] datas = new uint[dataLen]; foreach (BitInfo bif in bitInfos) { bif.encode(obj, datas); } return datas; } public void parse(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 > shiftB, field.FieldType)); } else { field.SetValue(obj, Convert.ChangeType((((int)(datas[idx] & mask)) > shiftB, field.FieldType)); } } } public class ArrayConverter { public static T[] convert(uint[] val) { return convert(val); } public static T1[] convert(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(); 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[] vals) { StringBuilder sb = new StringBuilder(); int width = typeWidth(); 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() { 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; } }
and the usage:
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; } }
