double check locking in singleton pattern

Question

it may be basic question

to have a singleton in multi-threaded environment we can use a lock. Please refer the code snippet. But why do we need double-checked locking in singleton pattern? And more what does double-checked locking means?

class singleton {     private static singleton instance = null;     private static singleton() { }      private static object objectlock = new object();      public static singleton Instance     {         get         {              lock (objectlock) //single - check lock             {                 if (instance == null)                 {                     instance = new singleton();                 }                  return instance;             }         }      } } 

Answer 1

Jon Skeet explains this in detail.

Locks are expensive.
If the object already exists, there's no point in taking out a lock.
Thus, you have a first check outside the lock.

However, even if the object didn't exist before you took the look, another thread may have created it between the if condition and the lock statement.
Therefore, you need to check again inside the lock.

However, the best way to write a singleton is to use a static constructor:

public sealed class Singleton {     private Singleton()     {     }      public static Singleton Instance { get { return Nested.instance; } }      private class Nested     {         // Explicit static constructor to tell C# compiler         // not to mark type as beforefieldinit         static Nested()         {         }          internal static readonly Singleton instance = new Singleton();     } }