C# assign value to another object member that is list

Question

I have question about assigning value to the list<> member of another object. I'm receiving error saying that the list<> member is NULL reference. It seems that i have to instantiate the list<> member?

public class Person
{
   public string FirstName {set;get:}
   public string LastName {set;get;}
   public string Country {set;get;
   public List<string> Hobbies;

}

public class Survey
{
   public List<Person> Poll;

   public void StartPoll()
   {       
       Person p = new Person();
       p.FullName = "Billy";
       p.LastName = "Bob";
       p.Location = "America";
       p.Hobbies.Add("Hiker");// this is where error occurs
       p.Hobbies.Add("Musician");// this is where error occurs

       Poll = new List<Person>();
       Poll.Add(p);

   }

}

I'd also strongly recommend that you turn `Hobbies` from a public field into a property. — Jon Skeet, Jan 28 '20 at 20:44
`p.Hobbies = new List() {"Hiker", "Musician"};` Or turn `Hobbies` into readonly property: `public List Hobbies {get;} = new List();` — Dmitry Bychenko, Jan 28 '20 at 20:44

score 1 · Accepted Answer · answered Jan 28 '20 at 20:46

You need to instantiate Hobbies collection.

public class Survey
{

   public List<Person> Poll;
   public Person p;

   public void StartPoll()
   {
       p = new Person();
       p.FullName = "Billy";
       p.LastName = "Bob";
       p.Location = "America";
       p.Hobbies = new List<string>(); // This line is missing in your code.
       p.Hobbies.Add("Hiker");// this is where error occurs
       p.Hobbies.Add("Musician");// this is where error occurs

       Poll = new List<Person>();
       Poll.Add(p);

   }

}

Or modify your Person class as below:

public class Person
{
   public Person()
   {
        this.Hobbies = new List<string>(); // Since you are instantiating here, you don't need to instantiate again in Survey or wherever/whenever you instantiate Person object in your application.
   }
   public string FirstName {set;get:}
   public string LastName {set;get;}
   public string Country {set;get;
   public List<string> Hobbies; // Modify this to property public List<string> Hobbies {get; set;}

}

C# assign value to another object member that is list

1 Answers1