Why Collection Initialization Throws NullReferenceException

Question

The following code throws a NullReferenceException :

internal class Foo
{
    public Collection<string> Items { get; set; } // or List<string>
}

class Program
{
    static void Main(string[] args)
    {
        new Foo()
            {
                Items = { "foo" } // throws NullReferenceException
            };
    }
}

1. Why don't collection initiliazers work in this case, although Collection<string> implements the Add() method, and why is NullReferenceException is thrown?
2. Is it possible to get the collection initializer working, or is Items = new Collection<string>() { "foo" } the only correct way to initialize it?

Answer 1

Thanks guys. As summary collection initializer doesn't create instance of collection itself, but just uses Add() to add items to existant instance , and throws NullReferenceException if instance doesn't exist

1.

internal class Foo
{    
    internal Foo()
    {
        Items  = new Collection<string>();
    }
    public Collection<string> Items { get; private set; }
}

var foo = new Foo()
                {
                    Items = { "foo" } // foo.Items contains 1 element "foo"
                };

2.

   internal class Foo
    {    
        internal Foo()
        {
            Items  = new Collection<string>();
            Items.Add("foo1");
        }
        public Collection<string> Items { get; private set; }
    }

    var foo = new Foo()
                    {
                        Items = { "foo2" } // foo.Items contains 2 elements: "foo1", "foo2"
                    };

Answer 2

In your Foo constructor you want to initialize collection.

internal class Foo
{
    public Foo(){Items = new Collection(); }
    public Collection<string> Items { get; set; } // or List<string>
}

class Program
{
    static void Main(string[] args)
    {
        new Foo()
            {
                Items = { "foo" } // throws NullReferenceException
            };
    }
}

Answer 3

You never instantiated Items . Try this.

new Foo()
    {
        Items = new Collection<string> { "foo" }
    };

To answer your second question: You need to add a constructor and initialize Items over there.

internal class Foo
{    
    internal Foo()
    {
        Items  = new Collection<string>();
    }
    public Collection<string> Items { get; private set; }
}

Why your code throws NullReferenceException ?

Answer 4

Foo.Items is declared, but an instance of Collection has never been assigned, so .Items is null .

Fix:

internal class Foo
{
    public Collection<string> Items { get; set; } // or List<string>
}

class Program
{
    static void Main(string[] args)
    {
        new Foo()
            {
                Items = new Collection<string> { "foo" } // no longer throws NullReferenceException :-)
            };
    }
}