Reasons to use private class

Question

When I started in the area, in a course of POO (Object Oriented Programming) the teacher explained about access modifiers...

I remember that on this very day he said that it would be possible to create a private class, but he saw no reason for it.

Why is it possible to declare classes private? Can I use them somehow? If so, how?

Obs.: I ask with no intention of breadth. If you consider too broad, please give me examples...

Answer 1

Private class is very little useful because it is available only within another class, that is, only the external class can instantiate an object of the inner class that is private. Not even returning an object of this type outside the class is possible because the type does not exist outside the class.

When the type is really only useful internally there is no reason to let the class be accessible from outside. The smaller the scope, the less errors you may have in your code, the less pollution namespace will have, so it is easier to locate what you want.

There is little reason, but there are cases for its use. This is done to hide implementation details. So one day you can change whatever you want without worrying. Everything that is exposed out of class becomes a contract that you have to respect under penalty of breaking existing codes.

A simplified and unnecessary example in this context, just to illustrate:

public class Classe {
    Temp temp = new Temp();
    public string Texto { get => temp.Str; set => temp.Str = value; }
    public int Valor { get => temp.I; set => temp.I = value; }

    private class Temp {
        public string Str { get; set; }
        public int I { get; set; }
    }
}

I put in the Github for future reference.

You cannot create an object of the type Temp out of this class.

One thing few people know is that the standard of the class is internal, that is, it is only accessible within the file itself Assembly code. To make the class accessible to every application you need to use the attribute public in it.

In other languages an internal class can have other functions, and if you try to learn from them, you won’t be learning how it works in C# which is just that.