binding to abstraction collection of abstraction class

Question

hi , i have :

public class Tangent
{
 public IPoint2d StartPoint {get; set;}
...
}

...

public IList<ITangent> Tangents = new ObservableCollection<ITangent>();
Tangents.Add(new Tangent(new Point2d()));

...


    <ListBox 
                   ItemsSource=
                    "{Binding Tangents,
                        UpdateSourceTrigger=PropertyChanged}">
                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <TextBlock 
                                Text="{Binding StartPoint,
                                UpdateSourceTrigger=PropertyChanged}"/>
                        </DataTemplate>
                    </ListBox.ItemTemplate>
                </ListBox>

the binding doesn't work.
any help would be appreciated,
i'm using wpfCore3.1 vs2019

Answer 1

Hi, the binding doesn't work because Tangents is not property. Try following demo:

<Window x:Class="WpfApp1.Window04"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:WpfApp04"
        mc:Ignorable="d"
        Title="Window04" Height="450" Width="800">
  <Window.DataContext>
    <local:ViewModel/>
  </Window.DataContext>
  <Grid>
    <ListBox ItemsSource="{Binding Tangents}">
      <ListBox.ItemTemplate>
        <DataTemplate>
          <TextBlock Text="{Binding StartPoint.Description, UpdateSourceTrigger=PropertyChanged}"/>
        </DataTemplate>
      </ListBox.ItemTemplate>
    </ListBox>
  </Grid>
</Window>

---

using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Windows;

namespace WpfApp04
{
  public class ViewModel
  {
    public ViewModel()
    {
      Tangents.Add(new Tangent(new Point2d()));
    }
    public IList<ITangent> Tangents { get; set; } = new ObservableCollection<ITangent>();
  }

  public class Tangent : ITangent
  {
    public Tangent(Point2d pt) => StartPoint = pt;
    public IPoint2d StartPoint { get; set; }
  }

  public interface ITangent
  {
    IPoint2d StartPoint { get; set; }
  }

  public class Point2d : IPoint2d
  {
    public string Description { get; set; } = "new Description";
  }

  public interface IPoint2d
  {
    string Description { get; set; }
  }
}

Answer 2

Is Tangents a property? You can only bind to properties. The code posted makes it look like a field

    public IList<ITangent> Tangents = new ObservableCollection<ITangent>();

StartPoint is a class Point2d I think you need to bind to one of the properties in the Point2d class

                         <TextBlock 
                             Text="{Binding StartPoint.MyProperty,
                             UpdateSourceTrigger=PropertyChanged}"/>