How to lazy initialize generic List<T> via class extension

Question

I'm trying to write safeAdd extention function for List class, so if it's not initialized yet - initialize it and add new value. But after I return from extension method, my just initialized list equals null. What's wrong?

Test class:

private class Test 
{
    public Test()
    {
        Id = Guid.NewGuid();
        //items = new List<string>();
    }
    public Guid Id { get; set; }
    public List<string> items { get; set; }
}

Extensions class:

public static class Helpers
{
    public static void safeAdd<T>(this List<T> list, T item)
    {
        if (list == null)
            list = new List<T>();
        list.Add(item);
    }
}

Part of main:

Test t = new Test();
t.items.safeAdd("testWord");
//Here t.items == null; WHY?

Answer 1

You have only assigned to a local method variable (which exists only inside the extension method) - this doesn't do anything to invoke the set, and to be honest there's no convenient way to get access to both the get and set without being verbose, or using lots of reflection / Expression code. IMO just do it in the class:

private List<string> items;
public List<string> Items { get {
     return items ?? (items = new List<string>()); } }

This will automatically initialize and assign the list the first time it is accessed.

It wouldn't be thread-safe unless I sync'd, or (perhaps preferable) used Interocked.CompareExchange for the assign. View  Interlocked it could be made thread-safe easily enough, but this is rarely a requirement for instance methods, and has associated overhead