Use an interface constraint : Interface constraint « Generic « C# / CSharp Tutorial

1. C# / CSharp Tutorial
2. Generic
3. Interface constraint

using System; 
 
class NotFoundException : ApplicationException { } 
 
public interface IUserID { 
  string Number { 
    get; 
    set; 
  } 
 
  string Name { 
    get; 
    set; 
  } 
} 
 
class Engineer : IUserID { 
  string name; 
  string number; 
 
  public Engineer(string n, string num) { 
    name = n; 
    number = num; 
  } 
 
  // Implement IUserID 
  public string Number { 
    get { return number; } 
    set { number = value; } 
  } 
 
  public string Name { 
    get { return name; } 
    set { name = value; } 
  } 
} 
 
class Manager : IUserID { 
  string name; 
  string number; 
 
  public Manager(string n, string num) { 
    name = n; 
    number = num; 
  } 
 
  // Implement IUserID 
  public string Number { 
    get { return number; } 
    set { number = value; } 
  } 
 
  public string Name { 
    get { return name; } 
    set { name = value; } 
  } 
} 
 
class Guest { 
 
} 
 
class IDList<T> where T : IUserID { 
  T[] idList; 
  int end; 
 
  public IDList() {  
    idList = new T[10]; 
    end = 0; 
  } 
 
  public bool add(T newEntry) { 
    if(end == 10) 
       return false; 
 
    idList[end] = newEntry; 
    end++; 
 
    return true; 
  } 
 
  public T findByName(string name) { 
 
    for(int i=0; i<end; i++) { 
      if(idList[i].Name == name)  
        return idList[i]; 
    } 
    throw new NotFoundException(); 
  } 
 
  public T findByNumber(string number) { 
    for(int i=0; i<end; i++) { 
      if(idList[i].Number == number)  
        return idList[i]; 
    } 
    throw new NotFoundException(); 
  } 
} 
 
class MainClass { 
  public static void Main() { 
    IDList<Engineer> plist = new IDList<Engineer>(); 
    plist.add(new Engineer("T", "1")); 
    plist.add(new Engineer("G", "6")); 
    plist.add(new Engineer("M", "9")); 
 
    try { 
      Engineer frnd = plist.findByName("G"); 
      Console.Write(frnd.Name + ": " + frnd.Number); 
    } catch(NotFoundException) { 
      Console.WriteLine("Not Found"); 
    } 
 
    Console.WriteLine(); 
 
    IDList<Manager> plist2 = new IDList<Manager>(); 
    plist2.add(new Manager("H", "8")); 
    plist2.add(new Manager("C", "2")); 
    plist2.add(new Manager("N", "4")); 
 
    try { 
      Manager sp = plist2.findByNumber("4"); 
      Console.WriteLine(sp.Name + ": " + sp.Number); 
    } catch(NotFoundException) { 
        Console.WriteLine("Not Found"); 
    } 
 
    // The following declaration is invalid 
    // because Guest does NOT implement IUserID. 
    // IDList<Guest> plist3 = new IDList<Guest>(); // Error! 
  } 
}

G: 6
N: 4