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singleton pattern is a dedin pattern where you restrict the instantiation of a class to single object. Used when you need exactly one object of the class.

Here is the sample C# code for Singleton Class
public sealed class Singleton
{
static readonly Singleton instance=new Singleton();

// Explicit static constructor to tell C# compiler
// not to mark type as beforefieldinit
static Singleton()
{
}

Singleton()
{
}

public static Singleton Instance
{
get
{
return instance;
}
}
}

when you want restrict on the instantiation of a class to single object, singleton design pattern is used.

Most of the time singleton class is also used in a multi threaded system. So it is better to protect the body of the construction with a mutex so that its thread safe