我只回答“如何”的问题。

这里有很多答案，但我想提出一种“设计模式”来实现这一功能。我将使用简单的语言机制，包括:

接口 嵌套类

例如，我们有两个主要的类:学生和大学。学生的GPA只有大学才允许获得。代码如下:

public interface IStudentFriend
{
    Student Stu { get; set; }
    double GetGPS();
}

public class Student
{
    // this is private member that I expose to friend only
    double GPS { get; set; }
    public string Name { get; set; }

    PrivateData privateData;

    public Student(string name, double gps) => (GPS, Name, privateData) = (gps, name, new PrivateData(this);

    // No one can instantiate this class, but Student
    // Calling it is possible via the IStudentFriend interface
    class PrivateData : IStudentFriend
    {
        public Student Stu { get; set; }

        public PrivateData(Student stu) => Stu = stu;
        public double GetGPS() => Stu.GPS;
    }

    // This is how I "mark" who is Students "friend"
    public void RegisterFriend(University friend) => friend.Register(privateData);
}

public class University
{
    var studentsFriends = new List<IStudentFriend>();

    public void Register(IStudentFriend friendMethod) => studentsFriends.Add(friendMethod);

    public void PrintAllStudentsGPS()
    {
        foreach (var stu in studentsFriends)
            Console.WriteLine($"{stu.Stu.Name}: stu.GetGPS()");
    }
}

public static void Main(string[] args)
{
    var Technion = new University();
    var Alex     = new Student("Alex", 98);
    var Jo       = new Student("Jo", 91);

    Alex.RegisterFriend(Technion);
    Jo.RegisterFriend(Technion);
    Technion.PrintAllStudentsGPS();

    Console.ReadLine();
}

这种友谊也可以通过使用“代理”——一些内部类来模拟。考虑下面的例子:

public class A // Class that contains private members
{
  private class Accessor : B.BAgent // Implement accessor part of agent.
  {
    private A instance; // A instance for access to non-static members.
    static Accessor() 
    { // Init static accessors.
      B.BAgent.ABuilder = Builder;
      B.BAgent.PrivateStaticAccessor = StaticAccessor;
    }
    // Init non-static accessors.
    internal override void PrivateMethodAccessor() { instance.SomePrivateMethod(); }
    // Agent constructor for non-static members.
    internal Accessor(A instance) { this.instance = instance; }
    private static A Builder() { return new A(); }
    private static void StaticAccessor() { A.PrivateStatic(); }
  }
  public A(B friend) { B.Friendship(new A.Accessor(this)); }
  private A() { } // Private constructor that should be accessed only from B.
  private void SomePrivateMethod() { } // Private method that should be accessible from B.
  private static void PrivateStatic() { } // ... and static private method.
}
public class B
{
  // Agent for accessing A.
  internal abstract class BAgent
  {
    internal static Func<A> ABuilder; // Static members should be accessed only by delegates.
    internal static Action PrivateStaticAccessor;
    internal abstract void PrivateMethodAccessor(); // Non-static members may be accessed by delegates or by overrideable members.
  }
  internal static void Friendship(BAgent agent)
  {
    var a = BAgent.ABuilder(); // Access private constructor.
    BAgent.PrivateStaticAccessor(); // Access private static method.
    agent.PrivateMethodAccessor(); // Access private non-static member.
  }
}

如果只用于访问静态成员，则会简单得多。 这种实现的好处是，所有类型都在友谊类的内部作用域中声明，并且与接口不同，它允许访问静态成员。

通过使用c#中的接口，您应该能够完成在c++中使用“friend”的相同种类的事情。它要求您显式地定义在两个类之间传递的成员，这是额外的工作，但也可能使代码更容易理解。

如果有人有一个合理使用“朋友”的例子，不能用界面模拟，请分享!我想更好地理解c++和c#之间的区别。

我怀疑这与c#编译模型有关——在运行时构建IL和JIT编译。也就是说:c#泛型与c++泛型有本质区别的原因是一样的。

从。net 3开始就有了InternalsVisibleToAttribute，但我怀疑他们只是在单元测试兴起后才添加它来迎合测试程序集。我想不出还有什么其他理由来使用它。

它在组装层工作但它能完成内部不能完成的工作;也就是说，您希望分发一个程序集，但希望另一个非分布式程序集具有对该程序集的特权访问。

很正确地，他们要求好友程序集是强键的，以避免有人在你受保护的程序集旁边创建一个假装的好友。

不要再为这种限制找借口了。朋友是坏的，但内在是好的?它们是一样的东西，只是那个朋友让你更精确地控制谁可以访问谁不能访问。

这是强制封装范式吗?所以你必须写访问器方法，然后呢?你应该如何阻止所有人(除了类B的方法)调用这些方法?你不能，因为你也控制不了，因为你失去了“朋友”。

No programming language is perfect. C# is one of the best languages I've seen, but making silly excuses for missing features doesn't help anyone. In C++, I miss the easy event/delegate system, reflection (+automatic de/serialization) and foreach, but in C# I miss operator overloading (yeah, keep telling me that you didn't need it), default parameters, a const that cannot be circumvented, multiple inheritance (yeah, keep telling me that you didn't need it and interfaces were a sufficient replacement) and the ability to decide to delete an instance from memory (no, this is not horribly bad unless you are a tinkerer)