What are the real world applications of the singleton pattern?

Question

Duplicate

On design patterns: When should I use the singleton?

class Singleton
{
    private static Singleton instance;

    private Singleton() {}

             


        

Answer 1

Jon Skeet, THE_SKEET, has a great article exemplifying various ways to implement correctly (Thread safe, lazy, performant) the Singleton pattern.

Go read it here.

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By popular demand (er... downvote) I reproduce the Skeet version:

public sealed class Singleton
{
    Singleton()
    {
    }

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

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

        internal static readonly Singleton instance = new Singleton();
    }
}

Here, instantiation is triggered by the first reference to the static member of the nested class, which only occurs in Instance. This means the implementation is fully lazy, but has all the performance benefits of the previous ones. Note that although nested classes have access to the enclosing class's private members, the reverse is not true, hence the need for instance to be internal here. That doesn't raise any other problems, though, as the class itself is private. The code is a bit more complicated in order to make the instantiation lazy, however.