Question

I have two class library projects: DataAccessLibrary and ServiceLayerLibrary.
ServiceLayerLibrary needs access to the DataAccessLibrary‘s methods and properties, but other projects must not have access to DataAccessLibrary.

How can I achieve this?

Answer 1

Have your members in DataAcceessLibrary internal and make use of friend assemblies so that ServiceLibrary may access them.

The internal keyword (in C#) gives access to other classes within the same assembly. Friend is the equivalent keyword in VB.Net. However, if you want another assembly to have access to another assembly’s “internal” stuff (in C#), then you can use the method from the web link below called “Friend Assemblies” where it exposes one assembly’s “internal” stuff to another assembly. Nowhere in this process do you actually use a keyword friend or friendly in C#. It is simply what they have called this relationship where you say [assembly:InternalsVisibleTo(“MyAssembly”)]. This is useful in situations where you have one assembly that utilizes functionality from another assembly that you do not want to make public. You can also use this technique with a strongly-named assembly like so [assembly: InternalsVisibleTo(“MyAssembly, PublicKey=xXxXx”)].

Example:

using System.Runtime.CompilerServices;
using System;

[assembly: InternalsVisibleTo("ServiceLibrary")]

// The class is internal by default. 
class FriendClass
{
    public void Test()
    {
        Console.WriteLine("Sample Class");
    }
}

// Public class that has an internal method. 
public class ClassWithFriendMethod
{
    internal void Test()
    {
        Console.WriteLine("Sample Method");
    }
}    

You may also use an alternative signing approach which might be better if you need public members in your DataAccessLibrary. It can be achieved by using LinkCommand with StrongNameIdentityPermission ( see http://www.codeproject.com/Articles/339909/Limiting-the-accessibility-Another-way-of-Friend-A ).