Binding to a UserControl in WPF using C#

Question

Preface The control I am giving as an example is an sample work for a larger project. I have already had some help from the community on Stackoverflow ironing out some of the finer points of bindings within the control. The surprise has been that I am having an issue binding in the control's hosting form.

I have read and researched around DependencyProperty for a lot of hours. I was not a WPF developer at the start of the year but I am now covering the role because of a death in the business, and I accept this is a big hill to climb.

The question is what is missing here in my:

The hosting form's XAML code

The code behind

    public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        DataContext = new viewModelBinding();
        BeSelected =  new modelMain("Yellow", "#FFFFE0");
    }

    public modelMain BeSelected
    { 
        get { return ((viewModelBinding)DataContext).Selected; }
        set { ((viewModelBinding)DataContext).Selected = value; }
    }

}

The ViewModel

    public class viewModelBinding :ViewModelBase
{
    modelMain sel = new modelMain("Red", "#FF0000");
    public modelMain Selected
    {
        get { return sel; }
        set { SetProperty(ref this.sel, value, "Selected"); }
    }
}

The next section is the control itself.

The Model

    public class modelMain:ViewModelBase
{
    public modelMain(string colName, string hexval)
    {
        ColorName = colName;
        HexValue = hexval;
    }

    string colorName;
    public string ColorName
    {
        get { return colorName; }
        set { SetProperty(ref this.colorName, value, "ColorName"); }
    }

    string hexValue;
    public string HexValue
    {
        get { return hexValue; }
        set { SetProperty(ref this.hexValue, value, "HexValue"); }
    }
}

The ViewModel

    public class viewModelMain:ViewModelBase
{
    ObservableCollection val = new ObservableCollection();
    public ObservableCollection ColorsList
    {
        get { return val; }
        set { SetProperty(ref this.val, value, "Colors"); }
    }


    modelMain selectedColor;
    public modelMain SelectedColour
    {          
        get{return selectedColor;}
        set { SetProperty(ref this.selectedColor, value, "SelectedColour"); }
    }

    public void SetCurrentColor(modelMain col)
    {
        SelectedColour = this.val.Where(x => x.ColorName == col.ColorName).FirstOrDefault(); 
    }

    public viewModelMain()
    {
        val.Add(new modelMain("Red", "#FF0000"));
        val.Add(new modelMain("Blue", "#0000FF"));
        val.Add(new modelMain("Green", "#008000"));
        val.Add(new modelMain("Yellow", "#FFFFE0"));

        SelectedColour = new modelMain("Blue", "#0000FF");
    }
}

The UserControl XAML

The UserControl Code behind

    public partial class UserControl1 : UserControl
{
    public UserControl1()
    {
        InitializeComponent();
    }

    ObservableCollection colorList = new viewModelMain().ColorsList;
    public ObservableCollection ColorList
    {
        get { return colorList; }
        set { colorList = value; }
    }


    public static readonly DependencyProperty SelectedColorProperty = DependencyProperty.Register(
        "SelectedColor",
        typeof(modelMain),
        typeof(UserControl1),
        new FrameworkPropertyMetadata(
            null, 
            FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
            new PropertyChangedCallback(OnSelectedColorChanged),
            new CoerceValueCallback(CoerceSelectedColorCallback)));


    private static void OnSelectedColorChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        UserControl1 uc = (UserControl1)d;
        uc.SelectedColor = (modelMain)e.NewValue;
    }

    private static object CoerceSelectedColorCallback(DependencyObject d, object value)
    {
        return (modelMain)value;
    }


    public modelMain SelectedColor
    {
        get { return (modelMain)GetValue(SelectedColorProperty); }
        set { SetValue(SelectedColorProperty, value); }
    }

    private void cboValue_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        var dat = sender as ComboBox;
        SelectedColor = (modelMain)dat.SelectedValue;
    }

    private void UserControl_Loaded(object sender, RoutedEventArgs e)
    {
        //var dat = sender as ComboBox;
        ////SelectedColor = (modelMain)dat.SelectedValue;
        //SelectedColor = (modelMain)this.SelectedColor;
    }
}

Please note that in the code behind there is unused code but within the sample I have used then for placing break points

I understand that no DataContext should exist in the UserControl because it precludes one in the hosting form.

The Question I was expecting the this line would be sufficient in the hosting form.

But it does not seem to do what I expected. I can see the BeSelected be initialised and it is holding a value but when the form loads I am expecting the colour yellow to enter the UserControl's and set DependencyProperty SelectedColor. This is not happening why and how can I get it to happen?

J.H. · Accepted Answer

To get you example working, do the following (for the most part, implement what the commenters said):

The hosting form's XAML code

The Mode doesn't really matter since MainWindow doesn't implement INPC nor does it ever know when ((viewModelBinding)DataContext).Selected (and therefor, BeSelected) is changed. Actually, like Joe stated, OneWayToSource doesn't work... RelativeSource was needed because BeSelected is a property of the MainWindow - not MainWindow's DataContext.

modelMain

modelMain needs to implement IEquatable (like Janne commented). Why? Because BeSelected = new modelMain(...) creates a new modelMain which is not one of the items in the ComboBox's ItemsSource (ColorList). The new object may have the same property values as one of the items but that doesn't make them equal (different objects = different address in memory). IEquatable gives you the opportunity to override that.

public class modelMain : ViewModelBase, IEquatable
{
  ...
  public bool Equals(modelMain other)
  {
    return (HexValue == other.HexValue);
  }
}

viewModelMain's ColorList's setter is calling SetProperty with property name "Colors" when it should be "ColorsList". It's not being used so it doesn't stop your example from working but it's still wrong.

Binding to a UserControl in WPF using C#

Answers (1)

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