using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(error.campaignNotFound());

        Console.WriteLine(error.campaignNotFound("id1"));

    }

    public class error

    {

        public static messageDelegate campaignNotFound { get; } = message("Campaign not found");

    }

    public delegate string messageDelegate(string arg = null);

    public static Func<string, messageDelegate> message = base1 => (param => param != null ? $"{base1} {param}" : $"{base1}");

}