read return value from delegate

Question

I am not sure if I understood the usage of delegates correctly but I would like to read delegate return value in publisher class. The example is below with description.

//Publisher class
    public class ValidateAbuse
    {

    public delegate List<String> GetAbuseList();
    public static GetAbuseList Callback;

    public void Ip(string ip)
    {
    //   I would like to read GetAbuseList value (List<String>) here. How to do that?
    }

    }


//Subscriber class
    class Server
    {

        public static void Start()
        {
            ValidateAbuse.Callback = GetIpAbuseList;
            ValidateAbuse.Ip(MyIp);
        }

        private static List<string> GetIpAbuseList()
        {
            //return List<String> to ValidateAbuse class and use return value in public void Ip(string ip) method 
        }

Answer 1

public void Ip(string ip)
{
  if (Callback != null)
  {
    List<String> valueReturnedByCallback = Callback();
  }
}

Answer 2

Here's a version that does not use static for ValidateAbuse and that uses the built-in Func<T> delegate.

public class ValidateAbuse
{
    private Func<List<string>> callback;

    public ValidateAbuse(Func<List<string>> callback)
    {
        this.callback = callback;
    }

    public void Ip(string ip)
    {
        var result = callback();
    }
}

public class Server
{
    public static void Start()
    {
        var validateAbuse = new ValidateAbuse(GetIpAbuseList);
        validateAbuse.Ip(MyIp);
    }

    private static List<string> GetIpAbuseList()
    {
        //return List<string> to ValidateAbuse class and use return value in public void Ip(string ip) method 
    }
}

I recommend you avoid static since that gives you a global state, which could later give you coupling problems and also makes it hard for you to unit test.

The other answers given so far has a guard clause, checking Callback for null. Unless that is expected behaviour (that Callback is null) I would avoid this. It's better to crash early than to get hard to debug errors later on.

I would also try to make the Server non-static.