I have some questions regarding the the singleton pattern as documented here:
<http://msdn.microsoft.com/en-us/library/ff650316.aspx>

The following code is an extract from the article:

```
using System;

public sealed class Singleton
{
   private static volatile Singleton instance;
   private static object syncRoot = new object();

   private Singleton() {}

   public static Singleton Instance
   {
      get 
      {
         if (instance == null) 
         {
            lock (syncRoot) 
            {
               if (instance == null) 
                  instance = new Singleton();
            }
         }

         return instance;
      }
   }
}

```

Specifically, in the above example, is there a need to compare instance to null twice, before and after the lock? Is this necessary? Why not perform the lock first and make the comparison?

Is there a problem in simplifying to the following?

```
   public static Singleton Instance
   {
      get 
      {
        lock (syncRoot) 
        {
           if (instance == null) 
              instance = new Singleton();
        }

         return instance;
      }
   }

```

Is the performing the lock expensive?