What is the C# equivalent of friend?

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.