private members in interface

Question

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();
}

Answer 1

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