Defining a variable with Interface or with Class-CodePudding

this popped into my head recently:

We have an Interface:

interface IPerson { }

and a Class that implements said Interface:

class Person : IPerson{ }

Now, what is the preferred way of initializing a Person-Variable, this:

Person person = new Person();

or:

IPerson person = new Person();

Using Type Inference and Intellisense

var person = new Person();

reveals that person is of type Person, instead of IPerson. Also I was thinking of something like this:

List<IPerson> persons;

vs. this:

List<Person persons;

What is the preferred way of declaring a variable or defining a Collection in this case, also in terms of future refactorings etc. If this is a duplicate question, please tell.

Best regards

CodePudding user response：

Usually, you prefer to use interfaces. Suppose that you have a list of Person and you also have a kind of person, like SuperHero:

class SuperHero : IPerson{ }

In this case, SuperHero may inherit from Person but as an example we may suppose that has not sense. So you have 2 classes implementing IPerson.

If you have a variable of type Person, you can't manage SuperHeros. And if you have a List<Person> you only can store Person, not SuperHeros.

If you need work with any IPerson, you must use interfaces:

List<IPerson> list = new List<IPerson>();
list.Add(new Person());
list.Add(new SuperHero());

And a point about "var". If you use var, the compiler change var for the type that you use at right. You can force with a casting:

var person = (IPerson)new Person();

When you work with OLE automation and COM objects, in .NET you need an inplementation of the interfaces. That implementacion has the Class suffix. But you can work with the interfaces, without the Class suffix. More information here: https://docs.microsoft.com/es-es/archive/blogs/mshneer/vs-2010-compiler-error-interop-type-xxx-cannot-be-embedded-use-the-applicable-interface-instead