"Object reference not set to an instance of an object" error when trying to add objects to a list inside a class [duplicate]

Question

This question already has answers here:

What is a NullReferenceException, and how do I fix it? (26 answers)

Closed 9 years ago.

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

public class Unit
{
    public string Name { get; set; }
    public List<Group> 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 1

Your List<Group> 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<Group> Contains { get; set; }

    public Unit()
    {
        Contains = new List<Group>();
    }
}

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<Group> Groups { get; set; }

    public Unit()
    {
        Groups = new List<Group>();
    }
}

Answer 2

Your Contains list within User class never gets instantiated.

You may want to change your User class to this instead:

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

    List<Group> list = new List<Group>();

    public List<Group> Contains
    {
        get { return list; }
    }
}

As you can see the Contains property now only returns a list instance that can of course be modified but never reaffected. Likely, this is what you'll want.

Although this is a tad outside the scope of your question, I would suggest that you rename the Contains property to Groups instead.

Answer 3

You have initialized the Unit But Unit.Contains Does not initialize. Before Adding objects to Contains list it must be initialized.

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

Or

In your Unit class constructor you can initialize List here the code

public class Unit
{
public string Name { get; set; }
public List<Group> Contains { get; set; }
public Unit()
{
Contains = new List<Group>();
}
}

I would prefer the 2nd solution as it will make sure that whenever Unit is initialized Contains will be initialized automatically.