Question

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What is the need of private constructor in C#?

Hi,

I’ve seen plenty of classes in .NET which have private constructor (Stream is one of them I think). When would I want to model a class like this?

I keep thinking that if my class has no internal state/fields, then I can make it have a private constructor.

Am I on the right track with this idea? I can understand the use of a factory (I’ve run into the tipping point a few times), but not with a private constructor class.

Thanks

Answer 1

Private constructors are normally used for:

1. Constructor-Chaining – As targets for other constructors in the same class.
2. Controlling construction – Occasionally, all the constructors of a class are made private to:
   1. Enforce the singleton pattern.
   2. Only allow construction through static factory-methods.
   3. Only allow nested classes (if any) to inherit from it.

I’ve seen plenty of classes in .NET
which have private constructor (Stream
is one of them I think)

System.IO.Stream is not an example of any of these – it has a single, protected constructor (at least as of .NET 4.0) . Since it is an abstract class, it does not make sense for it to have public constructors.

I keep thinking that if my class has
no internal state/fields, then I can
make it have a private constructor.

In this case, consider creating a static class instead, providing no instance constructors at all (a static class can’t have instance constructors since instances of it cannot be created). Of course, there are cases when this may not be appropriate despite the lack of any state, such as when the class must implement an interface.