c# - "Object reference not set to an instance of an object" error when trying to add objects to a list inside a class

I have some custom classes defined that include lists of other classes like so:

public class Unit
{
    public string Name { get; set; }
    public List Contains { get; set; }

}

public class Group
{
    public string Name { get; set; }
    public Type Type { get; set; }
    public int Number { get; set; }
}

public static Type basic_1 = new Type() { Name = "basic_1", Number = 1, Cost = 13 };

The basic idea is that you have a unit which contains a list of groups, each with a different type. The type contains specific properties while the classes that contain them are organizational in nature.

I then try to start building out these classes like so:

Unit unit1 = new Unit() { Name = "Unit 1" };
Group group1 = new Group() { Name = "Group 1", Number = 10, Type = basic_1 };
unit1.Contains.Add(group1);

But here I receive the error (on the Add method) "Object reference not set to an instance of an object." Looking at the locals in the debugger I can see that the Unit, Group and Type were all created successfully and the group1 contains all the Type values it's supposed to, but unit1.Contains is null.

What am I doing wrong? FYI I've never done something like this before so I don't even know if it's possible, but it seems to work fine up until this point.

Answer

Your List Contains never gets initialized, so when you try to access it, you get a null reference exception. Essentially all null reference exceptions are the same, you're trying to use an object that's null.

In this case, let's just add a default constructor to initialize the list for us.

public class Unit

{
    public string Name { get; set; }

    public List Contains { get; set; }

    public Unit()
    {
        Contains = new List();
    }
}

By the way, Contains is a terrible name for a list. Contains is usually a function call, as it's a verb. Usually better to use a noun for a collection, such as Groups.

public class Unit
{
    public string Name { get; set; }

    public List Groups { get; set; }


    public Unit()
    {
        Groups = new List();
    }
}