Question

I have a class named WhatClass that has List field in it. I need to be able to read-only this field, so I used a get property to expose it to other objects.

public class WhatClass
{
    List<SomeOtherClass> _SomeOtherClassItems;

    public List<SomeOtherClass> SomeOtherClassItems { get { return _SomeOtherClassItems; } }
}

However it turns out that any object can call

WhatClass.SomeOtherClassItems.Add(item);

How can I prevent this?

Answer 1

As others have said, you are looking for the .AsReadOnly() extension method.

However, you should store a reference to the collection instead of creating it during each property access:

private readonly List<SomeOtherClass> _items;

public WhatClass()
{
    _items = new List<SomeOtherClass>();

    this.Items = _items.AsReadOnly();
}

public ReadOnlyCollection<SomeOtherClass> Items { get; private set; }

This is to ensure that x.Items == x.Items holds true, which could otherwise be very unexpected for API consumers.

Exposing ReadOnlyCollection<> communicates your intent of a read-only collection to consumers. Changes to _items will be reflected in Items.