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();
}
}