CODEWITHSUNDEEP
c#.netc#-8.0
SilverFang
SilverFang

Reputation: 43

private members in interface

is this possible to create a private members in interface in .NET? I heard that it is possible now but I my IDE is rejecting it:

public interface IAnimal
{
    void SetDefaultName(string name)
    {
        ChangeName(name);
    }

    private string defaultName = "NoName";

    private void ChangeName(string name)
    {
        defaultName = name;
    }
    void Breath()
    {
        Console.WriteLine($"Default - I'm {defaultName}. <Breathing sounds>");
    }

    void Sound();
}

Upvotes: 4

Views: 2698

Answers (4)

Pawel
Pawel

Reputation: 624

Yes! Now you can in C# 8.0 but it has to be a static member. Like this:

public interface IAnimal
{
    static void SetDefaultName(string name)
    {
        ChangeName(name);
    }

    private static string defaultName = "NoName";

    private static void ChangeName(string name)
    {
        defaultName = name;
    }
    void Breath()
    {
        Console.WriteLine($"Default - I'm {defaultName}. <Breathing sounds>");
    }

    void Sound();
}

But you need to keep in mind that static fields will be shared across the application. Changing the defaultName will result in changing it in every place where you are using IAnimal

Upvotes: 5

Gauravsa
Gauravsa

Reputation: 6528

Yes, with C# 8.0, you can have public, private and protected members.

For example, following works:

public interface ITest
{
   private SomeEnum EnumTy { get => SomeEnum.Value1; }
}

If a class implements an interface and tries to access the variable, they will get an error.

public class TestImpl : ITest
{
 ITest.EnumTy = SomeEnum.Value2; // gives an error
}

The members can also be protected. As a suggestion, going forward its good to have public before methods to indicate more readability (even though public is default).

A good article is here:

https://jeremybytes.blogspot.com/2019/11/c-8-interfaces-public-private-and.html

Upvotes: 1

Mostafa Mohamed Ahmed
Mostafa Mohamed Ahmed

Reputation: 649

The actual question should be : should i do (or be able to do) it or not ?

The whole idea of the interface thing is to provide abstraction .

private members usually are implementation details that the consumer of your code shouldn't care about or know they exists so they don't belong to your abstract layer (interface)

the interface is the facade that the consumer will interact with ur service/component thorough , it should contain only the methods that the consumer will call , this is why originally interfaces should have only public modifiers

not to mention that putting such members in the interface will force every implementation of it to implement these members even if they don't need it

from my opinion the only reason C# changed interfaces and made them allow implementation is to make up for the language lack of support for multiple class inheritance , but that does mean that you should miss use ur interfaces like that

Upvotes: 0

Sweeper
Sweeper

Reputation: 274433

private methods used to be prohibited in interfaces because interfaces are supposed to be contracts. They are a guarantee that "this class has the following methods and properties". Why would it be useful to guarantee that a class has a private method? It isn't useful, because no one else can call it!

In C# 8, this changed. You can now specify private interface default methods. Note that it has to be a default method, not the ones that doesn't have an implementation. Here's the docs stating that fact:

The syntax for an interface is relaxed to permit modifiers on its members. The following are permitted: private, protected, internal, public, virtual, abstract, sealed, static, extern, and partial.

...

It is an error for a private or sealed function member of an interface to have no body.

Here is a quote from the docs explaining why this is allowed:

Static and private methods permit useful refactoring and organization of code used to implement the interface's public API.

Upvotes: 2

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