Why check if a class variable is null before instantiating a new object in the constructor?

Question

With a previous team I worked with, whenever a new Service class was created to handle business logic between the data layer and presentation layer, something like the following was done:

class DocumentService
{
    public DocumentRepository DocumentRepository { get; set; }

    public DocumentService()
    {
         if (DocumentRepository == null) DocumentRepository = new DocumentRepository();
    }
}

I never quite understood why the check for null was there. If the constructor is being called, that means it HAS to be null..since it's a new instance, right?

Why would this be done? It seems to me like it is a redundant step, but I don't want to miss anything and pass it off as bad practice.

Answer 1

In this exact context: Yes it is redundant.

There is no immediate reason for this code, it could be a left-over from an older method or anticipating on an implementation with multiple constructors. But I would not recommend using this 'pattern' or even keeping this code.