Use an interface constraint : Interface constraint : Generic C# Examples

    
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!  
    }  
}