What is the equivalent of a 'friend' keyword in C Sharp?

Question

What is the equivalent of a 'friend' keyword in C Sharp?

How do I use the 'internal' keyword?

I have read that 'internal' keyword is a replacement for 'friend' in C#.

I am using a DLL in my C# project that I have the source code for and yet I do not want to modify the existing code. I have inherited the class and I can use my inherited class any way I want. The problem is that most of the code in the parent class has protected methods. Will using a friend somehow make it possible to access or call these protected methods?

Solution 1

1. You can use the keyword access modifier internal to declare a type or type member as accessible to code in the same assembly only.

2. You can use the InternalsVisibleToAttribute class defined in System.Rutime.CompilerServices to declare a type as accessible to code in the same assembly or a specified assembly only.

You use the first as you use any other access modifier such as private. To wit:

internal class MyClass {
    ...
}

You use the second as follows:

[assembly:InternalsVisibleTo("MyFriendAssembly", PublicKey="...")]
internal class MyVisibleClass {
    ...
}

Both of these can rightly be considered the equivalent of friend in C#.

Methods that are protected are already available to derived classes.

Solution 2

No, "internal" is not the same as "friend" (at least the C++ 'friend')

friend specifies that this class is only accessible by ONE, particular class.
internal specifies that this class is accessible by ANY class in the assembly.

Solution 3

1. internal is the C# equivalent of the VB.NET friend keyword, as you have guessed (as opposed to a replacement)

2. Usage is as follows

   internal void Function() {}
   internal Class Classname() {}
   internal int myInt;
   internal int MyProperty { get; set; }
   

3. It, basically, is an access modifier that stipulates that the accessibility of the class / function / variable / property marked as internal is as if it were public to the Assembly it is compiled in, and private to any other assemblies

Solution 4

Your subclass will be able to access the protected members of the class you inherit.

Are you looking to give access to these protected members to another class?

Solution 5

Here's a weird trick I used for adding behaviour akin to C++'s friend keyword. This only works for nested classes AFAIK.

1. Create a nested protected or private interface with the variables you'd like to give access to via properties.
2. Let the nested class inherit this interface and implement it explicitly.
3. Whenever using an object of this nested class, cast it to the interface and call the respective properties.

Here's an example from Unity.

using System;
using UnityEngine;
using UnityEngine.Assertions;

namespace TL7.Stats
{
    [CreateAssetMenu(fileName = "Progression", menuName = "TL7/Stats/New Progression", order = 0)]
    public class Progression : ScriptableObject
    {
        // Provides access to private members only to outer class Progression
        protected interface IProgressionClassAccess
        {
            CharacterClass CharacterClass { get; set; }
        }

        [System.Serializable]
        public struct ProgressionClass : IProgressionClassAccess
        {
            [Header("DO NOT EDIT THIS VALUE.")]
            [SerializeField] private CharacterClass characterClass;
            [Tooltip("Levels are 0 indexed.")]
            [SerializeField] float[] healthOverLevels;

            public float[] HealthOverLevels => healthOverLevels;

            CharacterClass IProgressionClassAccess.CharacterClass
            {
                get => characterClass;
                set => characterClass = value;
            }
        }

        static readonly Array characterClasses = Enum.GetValues(typeof(CharacterClass));
        [SerializeField] ProgressionClass[] classes = new ProgressionClass[characterClasses.Length];

        public ProgressionClass this[in CharacterClass index] => classes[(int)index];

        void Awake()
        {
            for (int i = 0; i < classes.Length; ++i)
            {
                // Needs to be cast to obtain access
                (classes[i] as IProgressionClassAccess).CharacterClass = (CharacterClass)characterClasses.GetValue(i);
            }
        }

#if UNITY_EDITOR
        public void AssertCorrectSetup()
        {
            for (int i = 0; i < characterClasses.Length; ++i)
            {
                CharacterClass characterClass = (CharacterClass)characterClasses.GetValue(i);
                Assert.IsTrue(
                    (this[characterClass] as IProgressionClassAccess).CharacterClass == characterClass,
                    $"You messed with the class values in {GetType()} '{name}'. This won't do."
                );
            }
        }
#endif
    }
}

I think this only works for nested classes. In case you want to do this with regular classes, you'd need to nest them inside a partial outer class, which should work in theory, and use a protected or private nested interface (or two, if you're inclined) for providing them access to each other's privates... that came out wrong.

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Comments

[Dave Cousineau]

It's not equivalent, but it is the mechanism by which a set of classes work together without exposing themselves to the rest of the world. It also encourages smaller modules and reduces 'spaghetti' relationships between classes.