Singleton Design Pattern

Ensure a class has only one instance and provide a global point of access to it. Most objects in an application are responsible for their own work and operate on self-contained data and references that are within their given area of concern. However, there are objects that have additional responsibilities and are more global in scope, such as managing limited resources or monitoring the overall state of the system.

using System;
using System.Collections.Generic;

namespace Patterns
{

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");
}

// Next, load balance 15 requests for a server
LoadBalancer balancer = LoadBalancer.GetLoadBalancer();
for (int i = 0; i < 15; i++)
{
string serverName = balancer.NextServer.Name;
Console.WriteLine("Dispatch request to: " + serverName);
}

// Wait for user
Console.ReadKey();
}
}

/// <summary>
/// The 'Singleton' class
/// </summary>
sealed class LoadBalancer
{
// Static members are 'eagerly initialized', that is,
// immediately when class is loaded for the first time.
// .NET guarantees thread safety for static initialization
private static readonly LoadBalancer _instance =
new LoadBalancer();

// Type-safe generic list of servers
private List<Server> _servers;
private Random _random = new Random();

// Note: constructor is 'private'
private LoadBalancer()
{
// Load list of available servers
_servers = new List<Server>
{
new Server{ Name = "ServerI", IP = "120.14.220.18" },
new Server{ Name = "ServerII", IP = "120.14.220.19" },
new Server{ Name = "ServerIII", IP = "120.14.220.20" },
new Server{ Name = "ServerIV", IP = "120.14.220.21" },
new Server{ Name = "ServerV", IP = "120.14.220.22" },
};
}

public static LoadBalancer GetLoadBalancer()
{
return _instance;
}

// Simple, but effective load balancer
public Server NextServer
{
get
{
int r = _random.Next(_servers.Count);
return _servers[r];
}
}
}

/// <summary>
/// Represents a server machine
/// </summary>
class Server
{
// Gets or sets server name
public string Name { get; set; }

// Gets or sets server IP address
public string IP { get; set; }
}
}