What is the C# equivalent of friend? [duplicate]

Question

There's no direct equivalent of "friend" - the closest that's available (and it isn't very close) is InternalsVisibleTo. I've only ever used this attribute for testing - where it's very handy!

Example: To be placed in AssemblyInfo.cs

[assembly: InternalsVisibleTo("OtherAssembly")]

The closet equivalent is to create a nested class which will be able to access the outer class' private members. Something like this:

class Outer
{
    class Inner
    {
       // This class can access Outer's private members
    }
}

or if you prefer to put the Inner class in another file:

Outer.cs
partial class Outer
{
}


Inner.cs
partial class Outer
{
    class Inner
    {
       // This class can access Outer's private members
    }
}

Take a very common pattern. Class Factory makes Widgets. The Factory class needs to muck about with the internals, because, it is the Factory. Both are implemented in the same file and are, by design and desire and nature, tightly coupled classes -- in fact, Widget is really just an output type from factory.

In C++, make the Factory a friend of Widget class.

In C#, what can we do? The only decent solution that has occurred to me is to invent an interface, IWidget, which only exposes the public methods, and have the Factory return IWidget interfaces.

This involves a fair amount of tedium - exposing all the naturally public properties again in the interface.

There isn't a 'friend' keyword in C# but one option for testing private methods is to use System.Reflection to get a handle to the method. This will allow you to invoke private methods.

Given a class with this definition:

public class Class1
{
    private int CallMe()
    {
        return 1;
    }
}

You can invoke it using this code:

Class1 c = new Class1();
Type class1Type = c.GetType();
MethodInfo callMeMethod = class1Type.GetMethod("CallMe", BindingFlags.Instance | BindingFlags.NonPublic);

int result = (int)callMeMethod.Invoke(c, null);

Console.WriteLine(result);

If you are using Visual Studio Team System then you can get VS to automatically generate a proxy class with private accessors in it by right clicking the method and selecting "Create Unit Tests..."