IList<int> throws Null Reference Exception when adding values

Question

I have a class:

public class ClientModelData
{
    public int clientID { get; set; }
    public IList<int> LocationIDs { get; set; }
}

When I call it:

ClientModelData obj = new ClientModelData();
obj.LocationIDs.Add(1);

It throws an exception:

`((System.Collections.Generic.ICollection<int>)(client.LocationID))' is null`

Answer 1

LocationIDs is not initialized therefore it is giving you the error.

public IList<int> LocationIDs { get; set; }

You should create an instance in the constructor

public ClientModelData()
{
  LocationIDs = new List<int>();
}

Answer 2

You should initialize your list with actual object, for example in the constructor. Add this to your class:

public ClientModelData()
{
   LocationIDs = new List<int>();
}