set the default value of class property if it is null

Question

I've a class. How to set default value of the class properties to "na" in case they are null.

public class PharmacyOrder

{

public class MedicineRecord

{

   // how to set default value "na" for these properties

   public string MedicineId { get; set; }

   public string MedicineName { get; set; }

   public string MedicineDealer { get; set; }

}

}

this MedicineRecord is called from another class.

public PharmacyOrder.MedicineRecord SetMedicineRecord()

{

   PharmacyOrder.MedicineRecord obj = new PharmacyOrder.MedicineRecord();

obj.MedicineId = // assigned values

   // for other properties too

}

how to set default value of the properties to "na" to assign default value when null is passed?