Tutorial :How to make List's Add method protected, while exposing List with get property?

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?

Solution: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.

Solution:2

You're looking for the ReadOnlyCollection<T> class, which is a read-only wrapper around an IList<T>.

Since the ReadOnlyCollection<T> will reflect changes in the underlying list, you don't need to create a new instance every time.

For example:

public class WhatClass {      public WhatClass() {          _SomeOtherClassItems = new List<SomeOtherClass>();          SomeOtherClassItems = _SomeOtherClassItems.AsReadOnly();      }        List<SomeOtherClass> _SomeOtherClassItems;        public ReadOnlyCollection<SomeOtherClass> SomeOtherClassItems { get; private set; }  }

Solution:3

Use List<T>.AsReadOnly:

public ReadOnlyCollection<SomeOtherClass> SomeOtherClassItems  {      get      {          return _SomeOtherClassItems.AsReadOnly();      }  }

This will return a ReadOnlyCollection, which will throw an exception if a client calls Add through the interface. In addition, the ReadOnlyCollection type does not expose a public Add method.

Solution:4

How about using AsReadOnly()? - MSDN Documentation

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