Multi-Column combo box with filter over all columns

<window x:class="FilterableCombobox.MainWindow" xmlns:x="#unknown">
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    
    <window.resources>
        <style targettype="{x:Type ComboBox}">
            <style.setters>
                <setter property="ItemTemplate">
                    <setter.value>
                        <datatemplate>
                            <stackpanel orientation="Horizontal">
                            <label content="{Binding FirstCol}"></label>
                            <label content="{Binding SecondCol}"></label>
                            </stackpanel>
                        </datatemplate>
                    </setter.value>
                </setter>
            </style.setters>
        </style>
    </window.resources>
    
    <grid>
        <combobox itemssource="{Binding FilterItems}" iseditable="True" text="{Binding ComboText}"></combobox>
    </grid>
    
    
</window>

public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainWindowViewModel();
        }
    }

using System;
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.ComponentModel;
using System.Linq;
using System.Text;

namespace FilterableCombobox
{
    public class MyComboItem
    {
        public string FirstCol { get; set; }
        public string SecondCol { get; set; }
    }

    internal class MainWindowViewModel : INotifyPropertyChanged
    {
        private ObservableCollection<mycomboitem> _items;
        private ObservableCollection<mycomboitem> _filterItems;
        private string _comboText;

        public MainWindowViewModel()
        {
            _items = new ObservableCollection<mycomboitem>();
            _items.Add(new MyComboItem() { FirstCol = "1", SecondCol = "2" });
            _items.Add(new MyComboItem() { FirstCol = "A", SecondCol = "B" });
            _items.Add(new MyComboItem() { FirstCol = "3", SecondCol = "4" });
            _items.Add(new MyComboItem() { FirstCol = "C", SecondCol = "D" });
            FilterItems = _items;
        }


        public event PropertyChangedEventHandler PropertyChanged;

        public ObservableCollection<mycomboitem> FilterItems
        {
            get { return _filterItems; }
            set
            {
                _filterItems = value;
                InvokePropertyChanged("FilterItems");
            }
        }

        public string ComboText
        {
            get { return _comboText; }
            set
            {
                _comboText = value;
                InvokePropertyChanged("ComboText");
                if(string.IsNullOrWhiteSpace(_comboText))
                {
                    FilterItems = _items;
                }
                else
                {
                    FilterItems=new ObservableCollection<mycomboitem>(
                        from i in _items where i.FirstCol.Contains(value) || i.SecondCol.Contains(value)
                        select i
                        );
                }
            }
        }


        protected void InvokePropertyChanged(string prop)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null) handler(this, new PropertyChangedEventArgs(prop));
        }
    }
}