Use a property as factory method

Question

I have a base class `Base` which needs to create instances of another type `TRequired`, however, only derived classes from `Base` know how to construct those. Is it bad style to use an abstract property as factory method? e.g. ``` protected abstract TRequired NewTRequired { get; } ``` Should I use a method for some reason? Is there a guide why I should/shouldn't use a property here?

Accepted Answer

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

Or, if you prefer another perspective: two subsequent accesses of the member will return different results. A good rule of thumb is to use a method whenever this is the case, in order to not violate the principle of least astonishment.

This looks like it's reading the result of a variable, even though if you know that `NewTRequired` is a property (as opposed to a field) you also know that in reality it's running arbitrary code:

```
var prototype = Factory.NewTRequired;
```

I have deliberately put the result into a variable called `prototype` in order to better show that even an informed reader of this code can be easily thrown off: it would not be unreasonable to see this and think "right, so `NewTRequired` is the prototype object for X". That reader would certainly be astonished by the result of code like this:

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

Contrast this with a factory method. Now this code might give off a slight 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 astonish 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 have been followed but was not is the `DateTime.Now` property.

Use a property as factory method

Solution

Problem

Related problems