WPF Filter a ListBox

Question

I have a ListBox with binding to a list of strings. I want to filter the list when I enter text in a TextBox. How can I do it?

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Answer 1

If you set Dictionary as itemsource to listbox use the below code to sort,

    private void tb_filter_textChanged(object sender, TextChangedEventArgs e)
    {
        Dictionary<string, string> dictObject = new Dictionary<string, string>();
        ICollectionView view = CollectionViewSource.GetDefaultView(dictObject);
        view.Filter = CustomerFilter;
        listboxname.ItemsSource = view;
    }

    private bool CustomerFilter(object item)
    {
        KeyValuePair<string, string> Items = (KeyValuePair<string,string>) item;
        return Items.Value.ToString().Contains("a");
    }

The above code returns the items contaning "a".

Answer 2

Here is an attached property for binding a filter:

using System;
using System.Windows;
using System.Windows.Controls;

public static class Filter
{
    public static readonly DependencyProperty ByProperty = DependencyProperty.RegisterAttached(
        "By",
        typeof(Predicate<object>),
        typeof(Filter),
        new PropertyMetadata(default(Predicate<object>), OnByChanged));

    public static void SetBy(ItemsControl element, Predicate<object> value)
    {
        element.SetValue(ByProperty, value);
    }

    public static Predicate<object> GetBy(ItemsControl element)
    {
        return (Predicate<object>)element.GetValue(ByProperty);
    }

    private static void OnByChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        if (d is ItemsControl itemsControl &&
            itemsControl.Items.CanFilter)
        {
            itemsControl.Items.Filter = (Predicate<object>)e.NewValue;
        }
    }
}

Used like this in xaml:

<DataGrid local:Filter.By="{Binding Filter}"
          ItemsSource="{Binding Foos}">
    ...

And viewmodel:

public class ViewModel : INotifyPropertyChanged
{
    private string filterText;
    private Predicate<object> filter;

    public event PropertyChangedEventHandler PropertyChanged;

    public ObservableCollection<Foo> Foos { get; } = new ObservableCollection<Foo>();

    public string FilterText
    {
        get { return this.filterText; }
        set
        {
            if (value == this.filterText) return;
            this.filterText = value;
            this.OnPropertyChanged();
            this.Filter = string.IsNullOrEmpty(this.filterText) ? (Predicate<object>)null : this.IsMatch;
        }
    }

    public Predicate<object> Filter
    {
        get { return this.filter; }
        private set
        {
            this.filter = value;
            this.OnPropertyChanged();
        }
    }

    protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
    {
        this.PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
    }

    private bool IsMatch(object item)
    {
        return IsMatch((Foo)item, this.filterText);
    }

    private static bool IsMatch(Foo item, string filterText)
    {
        if (string.IsNullOrEmpty(filterText))
        {
            return true;
        }

        var name = item.Name;
        if (string.IsNullOrEmpty(name))
        {
            return false;
        }

        if (filterText.Length == 1)
        {
            return name.StartsWith(filterText, StringComparison.OrdinalIgnoreCase);
        }

        return name.IndexOf(filterText, 0, StringComparison.OrdinalIgnoreCase) >= 0;
    }
}

Answer 3

CollectionViewSource class can help here. As far as I can tell it has many capabilities to filter, sort and group collections.

ICollectionView view = CollectionViewSource.GetDefaultView(ElementList);
view.Filter = (o) => {return o;}//here is the lambda with your conditions to filter

When you don't need any filter just set view.Filter to null. Also check out this article on filtering