#Implementations of the Singleton in C-Sharp
##As written by Jon Skeet
####Note:
Jon's personal preference is solution 4.
Use solution 2 over solution 4 in cases where you need to be able to call other static methods without triggering initialization or you need to know whether or not the singleton has already been instantiated.
Solution 5 is 'elegant' but trickier than 2 or 4 while solution 6 is a simpler way to achieve laziness.
Don't use solution 1 because it's broken and solution 3 is not recommended because it has no benefits over solution 5.
###1 - Not Thread-Safe
// Bad code! Do not use!
public sealed class Singleton
{
private static Singleton instance=null;
private Singleton()
{
}
public static Singleton Instance
{
get
{
if (instance==null)
{
instance = new Singleton();
}
return instance;
}
}
}
###2 - Simple Thread-Safety
public sealed class Singleton
{
private static Singleton instance = null;
private static readonly object padlock = new object();
Singleton()
{
}
public static Singleton Instance
{
get
{
lock (padlock)
{
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
}
###3 - Attempted Thread-Safety Using Double-Check Locking
// Bad code! Do not use!
public sealed class Singleton
{
private static Singleton instance = null;
private static readonly object padlock = new object();
Singleton()
{
}
public static Singleton Instance
{
get
{
if (instance == null)
{
lock (padlock)
{
if (instance == null)
{
instance = new Singleton();
}
}
}
return instance;
}
}
}
###4 - Semi-Lazy yet Thread-Safe Without Locks
public sealed class Singleton
{
private static readonly Singleton instance = new Singleton();
// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Singleton()
{
}
private Singleton()
{
}
public static Singleton Instance
{
get
{
return instance;
}
}
}
###5 - Full Lazy Instantiation
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();
}
}
###6 - Using .NET 4 Lazy Type
public sealed class Singleton
{
private static readonly Lazy<Singleton> lazy =
new Lazy<Singleton>(() => new Singleton());
public static Singleton Instance { get { return lazy.Value; } }
private Singleton()
{
}
}
####Source: