Filter an ObservableCollection of object C#

Question

I want to filter an ObservableCollection of Person object by name for my Xamarin Form application. The goal is to filter this ObservableCollection to just display a part of it.

Here is my Person object class :

public class Person
{
    public string Name { get; set; }
    public string Address { get; set; }
    public string Phone { get; set; }
}

I tried to make a filter like this :

private ObservableCollection<Person> personItems = new ObservableCollection<Person>();

public ObservableCollection<Person> PersonItems
{
    get { return personItems; }
    set { personItems = value; OnPropertyChanged(); }
}

public void FilterPerson(string filter)
{
    if (string.IsNullOrWhiteSpace(filter))
    {
        PersonItems = personItems;
    }
    else 
    {
        PersonItems = personItems.Where((person) => person. Name.ToLower().Contains(filter));
        // Error here
    }
}

I have this error :

Cannot not explicitly convert type : 'System.Collections.Generic.IEnumerable' to 'System.Collections.ObjectModel.ObservableCollection

An observable collection is observable. you should remove unwanted items instead of replacing the complete collection with a new collection. — Sebastian Siemens, Aug 22 '16 at 12:13
Possible duplicate of [Cannot convert type IEnumerable to ObservableCollection...are you missing a cast?](http://stackoverflow.com/questions/14968948/cannot-convert-type-ienumerable-to-observablecollection-are-you-missing-a-cast) — Gilad Green, Aug 22 '16 at 12:14
there is also most likely missing code 'personItems = ', otherwise the error wouldnt appear. — mikus, Aug 22 '16 at 12:18
Never recreate the collection from new. https://www.youtube.com/watch?v=41NiKhFxYb0 — profimedica, Nov 05 '22 at 10:57

score 3 · Answer 1 · answered Aug 22 '16 at 12:31

Basically, there are two solutions:

If your PersonsItems list is not huge, you may recreate a whole collection each time a new filter string arrives. You don't even need an ObservableCollection in this case (due to the fact that you don't change the collection itself, you change a reference to a collection). All the UI elements will be recreated in this case
```
PersonItems = originalItems.Where((person) => person. Name.ToLower().Contains(filter)).ToList();
```
If your PersonsItems list is big enough, the first solution is not an option. In this case you need to manually call Add/Remove methods on the objects that should be added/removed. ObservableCollection has an imperative API and it fires an event each time Add/Remove is called. This event, in turn, can be observed by the ItemsControl that will make corresponding UI changes. Linq has a declarative API. That's why u need to sync a list to show with a filtered list manually.

As it was suggested above, you can also use a CollectionView that supports filtering. — Stanislav Khalash, Aug 22 '16 at 12:36
Ok thanks for the explanation, but the first solution is not working, the the or not the same why ? — fandro, Aug 22 '16 at 12:42
As I wrote, for the first approach you don't need an ObservableCollection. Just change PersonItems property/backing field type to List and use the assignment mentioned above. — Stanislav Khalash, Aug 22 '16 at 12:56

score 2 · Accepted Answer · answered Aug 22 '16 at 12:45

2

PersonItems = personItems.Where((person) => person. Name.ToLower().Contains(filter));

is returning IEnumerable.

replace it with PersonItems = new ObservableCollection<Person>(personItems.Where((person) => person.Name.ToLower().Contains(filter)));

You have to recreate the observable using the filtered results.

answered Aug 22 '16 at 12:45

Dimitri

6,923
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This completely beats the whole purpose of using ObservableCollection i.e. getting events out of it when things change so UI can react to the exact set of changes. – Matti Virkkunen Feb 09 '17 at 12:39
@Matti Virkkunen care to share your insights on alternative method instead of just downvoting an accepted answer? – Dimitri Feb 13 '17 at 14:23
1

You should never have to recreate your observable collection – Sam Debruyn Sep 28 '17 at 09:22
1

Sometimes updating the ObservableCollection is too expensive (high number of items bound to UI, UI flickers and/or becomes unresponsive) and recreation is the right approach. Besides, noone has provided a better way of filtering ObservableCollection (short of reverting to using ICollectionView) – Dimitri Oct 02 '17 at 12:26

score 1 · Answer 3 · answered Aug 22 '16 at 12:41

1

To do this in the past I've used James Montemagno's ObservableRangeCollection and Grouping helper functions. You can find them in this plugin https://github.com/jamesmontemagno/mvvm-helpers

answered Aug 22 '16 at 12:41

Richard Pike

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Good idea I use this plugin too – fandro Aug 22 '16 at 12:42

Filter an ObservableCollection of object C#

3 Answers3