Singleton Design Pattern

Sometimes you need to guarantee that only one instance of a class is created. One example might be a database object where you only want one connection. This class should be available globally, but should only have one instance. The Singleton creational pattern conforms to this requirement.

The O'Reilly book Head First Design Patterns By Eric and Elisabeth Freeman define the Singleton Pattern as a class that has only one instance and provides a global point of access to it.

The classic book Design Patterns by Gamma, Helm, Johnson and Vlissides (Gang of Four) add the following points.

Participants

• Singleton

1. Defines an Instance operation that lets clients access it unique instance. Instance is a class operation.
2. May be responsible for creating its own unique instance.

For my example, I'll create a singleton class in c# using the .Net Framework 4.0 that has a connection object in it. First, start a new console app project. Next, add a new class and name it Singleton.cs with following code...

namespace SingletonExample
{
public sealed class Singleton
{
private static readonly SqlConnection connection = new SqlConnection("Data Source=.;Initial Catalog=Chinook;Integrated Security=SSPI;");

static Singleton()
{
}

Singleton()
{
}

public static SqlConnection Connection
{
get
{
if (connection.State == ConnectionState.Closed) connection.Open();
return connection;
}
}
}
}

Now, in your program.cs file add the following code to Main...

static void Main(string[] args)
{
SqlConnection connection1 = singleton.connection;
SqlConnection connection2 = singleton.connection;
SqlConnection connection3 = singleton.connection;
SqlConnection connection4 = singleton.connection;
SqlConnection connection5 = singleton.connection;

Console.ReadLine();
}

As you step through the code run sp_who2 on the Chinook database and you will see only one connection added.

By the way, the Chinook database is on Codeplex and is designed as alternative to Northwind. It's small, will run on SQL Server, Oracle and MySQL and installs with a single script.