使ObservableCollection与Linq表达式的结果一起使用

Question

How to make a ObservableCollection working with the results of an Linq expression?

Ex:

items = new ObservableCollection<Item>(ctx.Items.Local.OrderBy(i => i.Name));
items.Add(new Item { Name = "ITEM NAME" });

At the time of saving the context, the above code does not work because:

var a items.Count; //a is 1
var b ctx.Items.Local.Count; //b is 0 --> Nothing added!!

Why A not equals B?? Can anyone help?

Obs. ctx is a DBContext

Answer 1

I'm not sure what you are hoping to happen. a being 1 and b being 0 is what I would expect from the code you have shared.

Check ObservableCollection's source code in http://referencesource.microsoft.com/#System/compmod/system/collections/objectmodel/observablecollection.cs

The constructor you are using will copy the contents of the list, so any further changes in items will not be reflected in the list used in its constructor, or in this case the IEnumerable.

public ObservableCollection(IEnumerable<T> collection)
{
    if (collection == null)
        throw new ArgumentNullException("collection");
    CopyFrom(collection);
}

private void CopyFrom(IEnumerable<T> collection)
{
    IList<T> items = Items;
    if (collection != null && items != null)
    {
        using (IEnumerator<T> enumerator = collection.GetEnumerator())
        {
            while (enumerator.MoveNext())
            {
                items.Add(enumerator.Current);
            }
        }
    }
}

If you want to listen to items changes, and reflect the changes in the base list, perhaps you can do this in items.CollectionChanged