How can I make a read-only ObservableCollection property?

Question

I\'d like to expose a property on a view model that contains a list of objects (from database).

I need this collection to be read-only. That is, I want to prevent A

Answer 1

You could change the type of your property to be an IEnumerable:

public IEnumerable<foo> CollectionOfFoo { 
     get { 
         return _CollectionOfFoo;
     }
}

I don't believe there is a standard interface that exposes an indexer. If you need it you could write an interface and extend ObservableCollection to implement it:

public interface IIndexerCollection<T> : IEnumerable<T>
{
    T this[int i]
    {
        get;
    }
}

public class IndexCollection<T> : ObservableCollection<T>, IIndexerCollection<T>
{
}

Answer 2

Use ReadOnlyObservableCollection< T >

public ReadOnlyObservableCollection<T> ReadOnlyFoo
{
    get { return new ReadOnlyObservableCollection<T> (_CollectionOfFoo); }
}

As has been pointed out, please use Eric J's answer as this one mistakenly is returning a new instance every time.

Answer 3

You could also override the list class that you're using and put an immutable flag in one of the constructors such that it will not add/remove if it was constructed with the immutable flag set to true.

Answer 4

The [previously] accepted answer will actually return a different ReadOnlyObservableCollection every time ReadOnlyFoo is accessed. This is wasteful and can lead to subtle bugs.

A preferable solution is:

public class Source
{
    Source()
    {
        m_collection = new ObservableCollection<int>();
        m_collectionReadOnly = new ReadOnlyObservableCollection<int>(m_collection);
    }
 
    public ReadOnlyObservableCollection<int> Items
    {
        get { return m_collectionReadOnly; }
    }
 
    readonly ObservableCollection<int> m_collection;
    readonly ReadOnlyObservableCollection<int> m_collectionReadOnly;
}

See ReadOnlyObservableCollection anti-pattern for a full discussion.

Answer 5

I don't like using ReadOnlyObservableCollection<T> as it seems like a mistake / broken class; I prefer a contract based approach instead.

Here is what I use that allows for covarience:

public interface INotifyCollection<T> 
       : ICollection<T>, 
         INotifyCollectionChanged
{}

public interface IReadOnlyNotifyCollection<out T> 
       : IReadOnlyCollection<T>, 
         INotifyCollectionChanged
{}

public class NotifyCollection<T> 
       : ObservableCollection<T>, 
         INotifyCollection<T>, 
         IReadOnlyNotifyCollection<T>
{}

public class Program
{
    private static void Main(string[] args)
    {
        var full = new NotifyCollection<string>();
        var readOnlyAccess = (IReadOnlyCollection<string>) full;
        var readOnlyNotifyOfChange = (IReadOnlyNotifyCollection<string>) full;


        //Covarience
        var readOnlyListWithChanges = 
            new List<IReadOnlyNotifyCollection<object>>()
                {
                    new NotifyCollection<object>(),
                    new NotifyCollection<string>(),
                };
    }
}