Collection ViewSource to filter listbox by typing in textbox

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C#

WPF

VS2012

I have a listbox in WPF where users firstname and lastname are listed. I have a textbox, and I am trying to filter as I type by the names. Here is what I am trying: (Nothing is being filtered when I am typing in the textbox)

Here is my VM

        #region Members

        
        private ObservableCollection<User> mUser = new ObservableCollection<User>();
        private CollectionViewSource usercvs = new CollectionViewSource();
        private string searchString;
        
        
        #endregion

        #region Properties

        public ObservableCollection<User> Users
        {
            get
            {
                return mUser;
            }
        }

        public string SearchFilter
        {
            get
            {
                return this.searchString;
            }

            set
            {
                if (!string.IsNullOrEmpty(this.searchString))
                    AddFilter();

                usercvs.View.Refresh(); 

                this.searchString = value;
            }
        }
        
        #endregion
      

        #region Methods
             
        private void AddFilter()
        {
            usercvs.Filter -= new FilterEventHandler(Filter);
            usercvs.Filter += new FilterEventHandler(Filter);

        }

        private void Filter(object sender, FilterEventArgs e)
        {
            // see Notes on Filter Methods:
            var src = e.Item as User;
            if (src == null)
                e.Accepted = false;
            else if (src.LastName != null && !src.LastName.Contains(SearchFilter))
                e.Accepted = false;
        }

        #endregion
    }
}