Private vs Static constructors in .Net

Question

I searched for this a lot, but none of the answers are clear (at-least for me!). Now I'm putting this question in SO, as I believe I can't get a more clarified answer anywhere else.

When should I use a private/static constructor in my class?

I'm fed up of usual answers, so please help me with some real-time examples and advantages/disadvantages of using these constructors.

Answer 1

Static constructors: used for initialising static members.

Private constructors: used when you only want a class to be instantiated from within its own code (typically in a static method). For example:

public class Thing
{
    static int Number;

    static Thing()
    {
        Number = 42; // This will only be called once, no matter how many instances of the class are created
    }

    // This method is the only means for external code to get a new Thing
    public static Thing GetNewThing()
    {
        return new Thing();
    }

    // This constructor can only be called from within the class.
    private Thing()
    {
    }
}

Answer 2

When should I use a private constructor in my class?

When you want a constructor, but don't want to expose it to the world. This could be because you have a factory method that calls the constructor (after validation), or because that constructor is called by ctor-chaining (i.e. public Foo(string) : this() { ...}).

Additionally, note that reflection code is often able to use a private constructor - for example serialization or ORM libraries.

Also, in early C# compilers, when you are writing what would now be a static class - having a private constructor was the only way of making it appear uncreatable.

When should I use a static constructor in my class?

When you need to initialize some static state prior to that state being accessed by instances or static methods.