Creating an Instance of an Interface

Question

I have the following interfaces defined:

public interface IAudit {
    DateTime DateCreated { get; set; }
}

public interface IAuditable {
    IAudit Audit { get; set; }
}

The IAuditable interface says which classes I will have an Audit for. The IAudit interface is the actual Audit for that class. For example say I have the following implementations:

public class User : IAuditable {
    public string UserName { get; set; }
    public UserAudit Audit { get; set; }
}

public class UserAudit : IAudit {
    public string UserName { get; set; }
    public DateTime DateCreated { get; set; }

    public UserAdit(User user) {
        UserName = user.UserName;
    }
}

Now given an object which is IAuditable (User from above), I'd like to be able to create an intance of IAudit (UserAdit from above) by feeding in the IAuditable object into the constructor. Ideally i'd have something like:

if (myObject is IAuditable) {
    var audit = new IAudit(myObject) { DateCreated = DateTime.UtcNow }; // This would create a UserAudit using the above example
}

However I have a bunch of problems:

• You can't create an instance of an interface
• No where in the code does it define which IAudit applies to which IAuditable
• I can't specify that the IAudit interface must have a constructor which takes an IAuditable

I'm sure this is a design pattern many have had before but I can't get my head around it. I'd really appreciate if someone could show me how this can be achieved.

Answer 1

You can't create an instance of an interface

Correct. You create an instance of an object implementing an interface:

IAuditable myUser = new User();

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No where in the code does it define which IAudit applies to which IAuditable

You can't do this directly with just one interface. You will need to rethink your design.

You can use a open generic type in the interface and implement it with closed types:

public interface IAudit<T> {
    DateTime DateCreated { get; set; }
}

public class UserAudit : IAudit<User> {
    public string UserName { get; set; }
    public DateTime DateCreated { get; set; }

    public UserAdit(User user) {
        UserName = user.UserName;
    }
}

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I can't specify that the IAudit interface must have a constructor which takes an IAuditable

Correct, you can't. See here. You need to create such a constructor on the implementers.