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关于C#生成MongoDB中ObjectId的实现方法

ObjectId介绍
在MongoDB中,文档(document)在集合(collection)中的存储需要一个唯一的_id字段作为主键。这个_id默认使用ObjectId来定义,因为ObjectId定义的足够短小,并尽最大可能的保持唯一性,同时能被快速的生成。

ObjectId 是一个 12 Bytes 的 BSON 类型,其包含
1.4 Bytes 自纪元时间开始的秒数
2.3 Bytes 机器描述符
3.2 Bytes 进程ID
4.3 Bytes 随机数

从定义可以看出,在同一秒内,在不同的机器上相同进程ID条件下,非常有可能生成相同的ObjectId。
同时可以根据定义判断出,在给定条件下,ObjectId本身即可描述生成的时间顺序

ObjectId的存储使用Byte数组,而其展现需将Byte数组转换成字符串进行显示,所以通常我们看到的ObjectId都类似于:

ObjectId("507f191e810c19729de860ea")

C#定义ObjectId类
代码如下:


View Code
   public class ObjectId
   {
     private string _string;

     public ObjectId()
     {
     }

     public ObjectId(string value)
       : this(DecodeHex(value))
     {
     }

     internal ObjectId(byte[] value)
     {
       Value = value;
     }

     public static ObjectId Empty
     {
       get { return new ObjectId("000000000000000000000000"); }
     }

     public byte[] Value { get; private set; }

     public static ObjectId NewObjectId()
     {
       return new ObjectId { Value = ObjectIdGenerator.Generate() };
     }

     public static bool TryParse(string value, out ObjectId objectId)
     {
       objectId = Empty;
       if (value == null || value.Length != 24)
       {
         return false;
       }

       try
       {
         objectId = new ObjectId(value);
         return true;
       }
       catch (FormatException)
       {
         return false;
       }
     }

     protected static byte[] DecodeHex(string value)
     {
       if (string.IsNullOrEmpty(value))
         throw new ArgumentNullException("value");

       var chars = value.ToCharArray();
       var numberChars = chars.Length;
       var bytes = new byte[numberChars / 2];

       for (var i = 0; i < numberChars; i += 2)
       {
         bytes[i / 2] = Convert.ToByte(new string(chars, i, 2), 16);
       }

       return bytes;
     }

     public override int GetHashCode()
     {
       return Value != null ? ToString().GetHashCode() : 0;
     }

     public override string ToString()
     {
       if (_string == null && Value != null)
       {
         _string = BitConverter.ToString(Value)
           .Replace("-", string.Empty)
           .ToLowerInvariant();
       }

       return _string;
     }

     public override bool Equals(object obj)
     {
       var other = obj as ObjectId;
       return Equals(other);
     }

     public bool Equals(ObjectId other)
     {
       return other != null && ToString() == other.ToString();
     }

     public static implicit operator string(ObjectId objectId)
     {
       return objectId == null ? null : objectId.ToString();
     }

     public static implicit operator ObjectId(string value)
     {
       return new ObjectId(value);
     }

     public static bool operator ==(ObjectId left, ObjectId right)
     {
       if (ReferenceEquals(left, right))
       {
         return true;
       }

       if (((object)left == null) || ((object)right == null))
       {
         return false;
       }

       return left.Equals(right);
     }

     public static bool operator !=(ObjectId left, ObjectId right)
     {
       return !(left == right);
     }
   }

C#实现ObjectId的生成器
代码如下:

View Code
   internal static class ObjectIdGenerator
   {
     private static readonly DateTime Epoch =
       new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
     private static readonly object _innerLock = new object();
     private static int _counter;
     private static readonly byte[] _machineHash = GenerateHostHash();
     private static readonly byte[] _processId =
       BitConverter.GetBytes(GenerateProcessId());

     public static byte[] Generate()
     {
       var oid = new byte[12];
       var copyidx = 0;

       Array.Copy(BitConverter.GetBytes(GenerateTime()), 0, oid, copyidx, 4);
       copyidx += 4;

       Array.Copy(_machineHash, 0, oid, copyidx, 3);
       copyidx += 3;

       Array.Copy(_processId, 0, oid, copyidx, 2);
       copyidx += 2;

       Array.Copy(BitConverter.GetBytes(GenerateCounter()), 0, oid, copyidx, 3);

       return oid;
     }

     private static int GenerateTime()
     {
       var now = DateTime.UtcNow;
       var nowtime = new DateTime(Epoch.Year, Epoch.Month, Epoch.Day,
         now.Hour, now.Minute, now.Second, now.Millisecond);
       var diff = nowtime - Epoch;
       return Convert.ToInt32(Math.Floor(diff.TotalMilliseconds));
     }

     private static byte[] GenerateHostHash()
     {
       using (var md5 = MD5.Create())
       {
         var host = Dns.GetHostName();
         return md5.ComputeHash(Encoding.Default.GetBytes(host));
       }
     }

     private static int GenerateProcessId()
     {
       var process = Process.GetCurrentProcess();
       return process.Id;
     }

     private static int GenerateCounter()
     {
       lock (_innerLock)
       {
         return _counter++;
       }
     }
   }

使用举例
代码如下:

class Program
   {
     static void Main(string[] args)
     {
       Console.ForegroundColor = ConsoleColor.Red;

       ObjectId emptyOid = ObjectId.Empty;
       Console.WriteLine(emptyOid);

       Console.WriteLine();
       Console.ForegroundColor = ConsoleColor.Green;

       for (int i = 0; i < 10; i++)
       {
         ObjectId oid = ObjectId.NewObjectId();
         Console.WriteLine(oid);
       }

       Console.WriteLine();
       Console.ForegroundColor = ConsoleColor.Blue;

       ObjectId existingOid;
       ObjectId.TryParse("507f191e810c19729de860ea", out existingOid);
       Console.WriteLine(existingOid);

       Console.ReadKey();
     }
   }

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