The Singleton design pattern is based on the concept of restricting the instantiation of a class to one object. Say one object needs to perform the role of a coordinator between various instances of the application that depend on a common object, we may design an application using a Singleton. Usage of Singleton patterns is common in Banking, Financial and Travel based applications where the singleton object consists of the network related information.
A singleton class may be used to instantiate an object of it, only if that object does not already exist. In case the object exists, a reference to the existing object is given. A singleton object has one global point of access to it.
An ASP.NET Web Farm is also based on the Singleton pattern. In a Web Farm, the web application resides on several web servers. The session state is handled by a Singleton object in the form of the aspnet_state.exe, that interacts with the ASP.NET worker process running on each web server. Note that the worker process is the aspnet_wp.exe process. Imagine one of the web servers shutting down, the singleton object aspnet_state.exe still maintains the session state information across all web servers in the web farm.
In .NET, in order to create a singleton, a class is created with a private constructor, and a "static readonly" variable as the member that behaves as the instance.   _{Your Comment}