c# - Where/How to instaniate an object to handle null reference exception -

hey guys kind of simple question, i'm not sure where/how initialize new object instance don't error. have class object(contact) has class object(contactinfo) , user decides not input(instantiate) contactinfo object. later when try search via contact.contactinfo, error. below have line of code error , have 2 classes:

foreach (var contact in contacts) {     if (string.equals(contact._contactinfo.city.tolower(), city, stringcomparison.currentculture))     {         contactsbycity.add(contact);     } } 

and 2 classes:

public class contact : person {     private contactinfo info;     private contactinfoloader loader;     public contactinfo _contactinfo { get; set; }      public contact()     { }     public contact(contactinfo _info)     {         info = _info;                }     public contactinfo getcontactinfo()     {         loader = new contactinfoloader(this);         return loader.gathercontactinfo();     }      }  public class contactinfo {     public string phonenumber { get; set; }     public string address { get; set; }     public string city { get; set; }     public string state { get; set;}      public contactinfo()     { }  } 

if want guarantee contactinfo info won't null after contact constructed, you'll need check @ time of construction. consider this:

public class contact : person {     private contactinfo info;     public contactinfo _contactinfo { get; set; }      public contact(contactinfo _info)     {         if (_info == null)             throw new argumentnullexception("_info");         info = _info;     }     public contact(contactinfoloader loader)         : this(loader.gathercontactinfo())     {     } } 

bonus:

a more standard style class definition equivalent semantics be

public class contact : person {     //auto-generates private field     public contactinfo info { get; private set; }      public contact(contactinfo info)     {         if (info == null)             throw new argumentnullexception("info");         this.info = info;     }     public contact(contactinfoloader loader)         : this(loader.gathercontactinfo())     {     } }