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Ensure a class has only one instance and provide a global point of access to it.
Frequency of use:  medium high
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The classes and/or objects participating in this pattern are:
- Singleton (LoadBalancer)
- defines an Instance operation that lets clients access its
unique instance. Instance is a class operation.
- responsible for creating and
maintaining its own unique instance.
This structural code demonstrates the Singleton pattern which assures only a single instance
(the singleton) of the class can be created.
Show code
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// Singleton pattern -- Structural example
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using System;
namespace DoFactory.GangOfFour.Singleton.Structural
{
// MainApp test application
class MainApp
{
static void Main()
{
// Constructor is protected -- cannot use new
Singleton s1 = Singleton.Instance();
Singleton s2 = Singleton.Instance();
if (s1 == s2)
{
Console.WriteLine("Objects are the same instance");
}
// Wait for user
Console.Read();
}
}
// "Singleton"
class Singleton
{
private static Singleton instance;
// Note: Constructor is 'protected'
protected Singleton()
{
}
public static Singleton Instance()
{
// Use 'Lazy initialization'
if (instance == null)
{
instance = new Singleton();
}
return instance;
}
}
}
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Output
Objects are the same instance
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This real-world code demonstrates the Singleton pattern as a LoadBalancing object.
Only a single instance (the singleton) of the class can be created because
servers may dynamically come on- or off-line and every request must
go throught the one object that has knowledge about the state of the (web) farm.
Show code
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// Singleton pattern -- Real World example
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using System;
using System.Collections;
using System.Threading;
namespace DoFactory.GangOfFour.Singleton.RealWorld
{
// MainApp test application
class MainApp
{
static void Main()
{
LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
LoadBalancer b4 = LoadBalancer.GetLoadBalancer();
// Same instance?
if (b1 == b2 && b2 == b3 && b3 == b4)
{
Console.WriteLine("Same instance\n");
}
// All are the same instance -- use b1 arbitrarily
// Load balance 15 server requests
for (int i = 0; i < 15; i++)
{
Console.WriteLine(b1.Server);
}
// Wait for user
Console.Read();
}
}
// "Singleton"
class LoadBalancer
{
private static LoadBalancer instance;
private ArrayList servers = new ArrayList();
private Random random = new Random();
// Lock synchronization object
private static object syncLock = new object();
// Constructor (protected)
protected LoadBalancer()
{
// List of available servers
servers.Add("ServerI");
servers.Add("ServerII");
servers.Add("ServerIII");
servers.Add("ServerIV");
servers.Add("ServerV");
}
public static LoadBalancer GetLoadBalancer()
{
// Support multithreaded applications through
// 'Double checked locking' pattern which (once
// the instance exists) avoids locking each
// time the method is invoked
if (instance == null)
{
lock (syncLock)
{
if (instance == null)
{
instance = new LoadBalancer();
}
}
}
return instance;
}
// Simple, but effective random load balancer
public string Server
{
get
{
int r = random.Next(servers.Count);
return servers[r].ToString();
}
}
}
}
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Output
Same instance
ServerIII
ServerII
ServerI
ServerII
ServerI
ServerIII
ServerI
ServerIII
ServerIV
ServerII
ServerII
ServerIII
ServerIV
ServerII
ServerIV
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This .NET optimized code demonstrates the
same code as above but uses more modern, built-in .NET features.
Here an elegant .NET specific solution is offered.
The Singleton pattern simply uses a private constructor and a static readonly instance
variable that is lazily initialized. Thread safety is guaranteed by
the compiler.
Show code
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// Singleton pattern -- .NET optimized
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using System;
using System.Collections;
namespace DoFactory.GangOfFour.Singleton.NETOptimized
{
// MainApp test application
class MainApp
{
static void Main()
{
LoadBalancer b1 = LoadBalancer.GetLoadBalancer();
LoadBalancer b2 = LoadBalancer.GetLoadBalancer();
LoadBalancer b3 = LoadBalancer.GetLoadBalancer();
LoadBalancer b4 = LoadBalancer.GetLoadBalancer();
// Confirm these are the same instance
if (b1 == b2 && b2 == b3 && b3 == b4)
{
Console.WriteLine("Same instance\n");
}
// All are the same instance -- use b1 arbitrarily
// Load balance 15 requests for a server
for (int i = 0; i < 15; i++)
{
Console.WriteLine(b1.Server);
}
// Wait for user
Console.Read();
}
}
// Singleton
sealed class LoadBalancer
{
// Static members are lazily initialized.
// .NET guarantees thread safety for static initialization
private static readonly LoadBalancer instance =
new LoadBalancer();
private ArrayList servers = new ArrayList();
private Random random = new Random();
// Note: constructor is private.
private LoadBalancer()
{
// List of available servers
servers.Add("ServerI");
servers.Add("ServerII");
servers.Add("ServerIII");
servers.Add("ServerIV");
servers.Add("ServerV");
}
public static LoadBalancer GetLoadBalancer()
{
return instance;
}
// Simple, but effective load balancer
public string Server
{
get
{
int r = random.Next(servers.Count);
return servers[r].ToString();
}
}
}
}
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Output
Same instance
ServerIV
ServerIV
ServerIII
ServerV
ServerII
ServerV
ServerII
ServerII
ServerI
ServerIV
ServerIV
ServerII
ServerI
ServerV
ServerIV
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