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

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?