Use property as a factory method

Question

I have a base class Base that should instantiate another type of TRequired , however, only derived classes from Base know how to build them.

Is it nice to use an abstract method as a factory method? eg.

 protected abstract TRequired NewTRequired { get; } 

Should I use the method for any reason? Is there any guide why I should / should not use the property here?

Answer 1

You should definitely use the method because accessing this member does something. Calling a method is a good way for the code to speak on its own in this regard.

Or, if you prefer a different perspective: two subsequent member accesses return different results. A good rule is to use the method whenever this occurs, so as not to violate the principle of least surprise .

This is like reading the result of a variable, even if you know that NewTRequired is a property (as opposed to a field), you also know that it actually executes arbitrary code:

 var prototype = Factory.NewTRequired; 

I intentionally put the result in a variable called prototype to better show that even an informed reader of this code can be easily discarded: it would be unreasonable to see this and think “right”, so NewTRequired is the prototype of the object for X. ”This reader was undoubtedly would be amazed at the result of this code:

 var eq = object.ReferenceEquals(prototype, Factory.NewTRequired); 

Contrast this with the factory method. Now this code may give a little smell:

 // hmmm... are we actually using this as a prototype? // because it sure looks like an instance created just for the occasion. var prototype = Factory.NewTRequired(); 

And this code will never surprise you:

 // obviously should be false, the code screams "I am creating new instances!" var eq = object.ReferenceEquals(Factory.NewTRequired(), Factory.NewTRequired()); 

A famous example of where this rule really should be followed, but was not , is DateTime.Now .

Answer 2

Instead, I would recommend a method:

 protected abstract TRequired CreateRequired(); 

Creation means "work." This is better suited for a method versus a property, since the getter property implies that it will usually be returned quickly without a lot of code.

Even the title property of your question as a factory method implies that the factory method must be a method.

Answer 3

Properties are for things that look like fields, such as the location of an object.

The property that returns a new instance every time you receive it is a very poor design.

You should use a method instead.