Two-Way Binding Issue of Unknown Object in WPF Custom Control Dependency Property

Question

I\'m having a Custom Control - Implemented for AutoComplete TextBox. I got all idea\'s from the following question Create a Custom Control with the combination of multiple c

Answer 1

First of all, this post would have fitted better in CodeReview.

Second, i can imagine, what you did want to do. To shorten things, i recommend you to not use generic collections in your case.

I've modified the Control a bit:

public class BTextBox : ItemsControl {

    static BTextBox() {
      DefaultStyleKeyProperty.OverrideMetadata(typeof(BTextBox), new FrameworkPropertyMetadata(typeof(BTextBox)));
    }

    private TextBox _textBox;
    private ItemsControl _itemsView;

    public static readonly DependencyProperty ProviderCommandProperty = DependencyProperty.Register("ProviderCommand", typeof(ICommand), typeof(BTextBox), new PropertyMetadata(null));
    public static readonly DependencyProperty AutoItemsSourceProperty = DependencyProperty.Register("AutoItemsSource", typeof(IEnumerable), typeof(BTextBox), new PropertyMetadata(null, OnItemsSourceChanged));

    public IEnumerable AutoItemsSource {
      get {
        return (IEnumerable)GetValue(AutoItemsSourceProperty);
      }
      set {
        SetValue(AutoItemsSourceProperty, value);
      }
    }

    private static void OnItemsSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
      var tb = d as BTextBox;
      if ((e.NewValue != null) && ((tb.ItemsSource as IList) != null)) {
        foreach (var item in e.NewValue as IEnumerable) {
          (tb.AutoItemsSource as IList).Add(item);
        }

      }
    }

    public override void OnApplyTemplate() {
      this._textBox = this.GetTemplateChild("PART_TextBox") as TextBox;
      this._itemsView = this.GetTemplateChild("PART_ListBox_Sugg") as ItemsControl;
      this._itemsView.ItemsSource = this.AutoItemsSource;
      this._textBox.TextChanged += (sender, args) => {
        this.ProviderCommand?.Execute(this._textBox.Text);
      };

      base.OnApplyTemplate();
    }

    public ICommand ProviderCommand {
      get {
        return (ICommand) this.GetValue(ProviderCommandProperty);
      }
      set {
        this.SetValue(ProviderCommandProperty, value);
      }
    }

    public ICommand AddCommand {
      get {
        return new RelayCommand(obj => {
          (this.ItemsSource as IList)?.Add(obj);
        });
      }
    }

  }

Then i've fixed your XAML to get thing to even compile and run:

At last a valuable remark:

Never ever allow setters on your ItemsSources. If you override them, the binding will break. Use .Clear() and .Add() instead as you see below:

public class StringModel : INotifyPropertyChanged {
    public event PropertyChangedEventHandler PropertyChanged;

    private readonly ObservableCollection _collection = new ObservableCollection();
    private readonly ObservableCollection _suggCollection = new ObservableCollection();
    private readonly ObservableCollection _primaryCollection = new ObservableCollection();

    public ObservableCollection Collection => this._collection;

    public ObservableCollection SuggCollection => this._suggCollection;

    public StringModel() {
      this._primaryCollection.Add("John");
      this._primaryCollection.Add("Jack");
      this._primaryCollection.Add("James");
      this._primaryCollection.Add("Emma");
      this._primaryCollection.Add("Peter");
    }

    public ICommand AutoBTextCommand {
      get {
        return new RelayCommand(obj => {
          this.Search(obj as string);
        });
      }
    }

    private void Search(string str) {
      this.SuggCollection.Clear();
      foreach (var result in this._primaryCollection.Where(m => m.ToLowerInvariant().Contains(str.ToLowerInvariant())).Select(m => m)) {
        this.SuggCollection.Add(result);
      }

    }

  }

Note

Sice i didnt have your DelegateCommand-implementation, i've used my RelayCommand instead. You can change it withour any issues. I think its the same thing but a different name for it.
You also might consider to display your suggestions right from the start. This might provide a better user-expierience, but thats just my opinion