friend

gistfile1.cs

// ApiA.cs
using PostSharp.Constraints;

namespace PostsharpArchitecturalConstraints.API.NamespaceA
{
    [Friend]
    internal class ApiA
    {
        public string GetFriendsName()
        {
            return "Mr. Friendly";
        }
    }
}

// ApiB.cs
using PostsharpArchitecturalConstraints.API.NamespaceA;

namespace PostsharpArchitecturalConstraints.API.NamespaceB
{
    public class ApiB
    {
        public string GetFriendsName()
        {
            var a = new ApiA();
            return a.GetFriendsName();
        }
    }
}