WPF. Assigning a value to the DataContext property doesn't change the value of the bound property

Question

Assigning a value to the DataContext property doesn't change the value of the bound property.

I need to get the value of a bound property multiple times for each record in a table to calculate the sum value for a column in a DataGrid.

For this I use a special dummy FrameworkElement object.

The object code looks like this:

  public class BindValueReader : FrameworkElement
  {
    public static readonly DependencyProperty ValueProperty =
      DependencyProperty.Register("Value",
                                  typeof(object),
                                  typeof(BindValueReader),
                                  new PropertyMetadata(null));

    public object Value
    {
      get { return GetValue(ValueProperty); }
      set { SetValue(ValueProperty, value); }
    }
  }

It is only used to get the value from the binding.

I use the following code to get the value of the bound property for each record.

  Decimal sumVal = 0;
  foreach (DataRowView rowView in view)
  {
    BindValueReader valueReader = new BindValueReader();
    valueReader.DataContext = rowView;
    valueReader.SetBinding(BindValueReader.ValueProperty, bdn);
    if (valueReader.Value != null)
      sumVal += (Decimal)valueReader.Value;
  }

it works correctly but is rather slow.

When I try to optimize the code like this

  Binding bdn = new Binding("Income");

  BindValueReader valueReader = new BindValueReader();
  valueReader.SetBinding(BindValueReader.ValueProperty, bdn);

  DataView view = dataTable.DefaultView;
  Decimal sumVal = 0;

  foreach (DataRowView rowView in view)
  {
    valueReader.DataContext = rowView;
    if (valueReader.Value != null)
      sumVal += (Decimal)valueReader.Value;
  }

it stops working.

I can't figure out why.

Here is the complete program code:

XAML:

<Window x:Class="EhLibTestApp.TestWindow1"
        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:EhLibTestApp"
        mc:Ignorable="d"
        Title="TestWindow1" Height="450" Width="800">
    <Grid>
    <StackPanel HorizontalAlignment="Stretch" Height="63" VerticalAlignment="Top" Orientation="Horizontal" >
      <Button Content="Button" Width="87" Margin="10" Click="Button_Click"/>
    </StackPanel>

  </Grid>
</Window>

CS:

using System;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Data;
using System.Diagnostics;

namespace EhLibTestApp
{
  /// <summary>
  /// Interaction logic for TestWindow1.xaml
  /// </summary>
  public partial class TestWindow1 : Window
  {
    public TestWindow1()
    {
      InitializeComponent();
    }

    private void Button_Click(object sender, RoutedEventArgs e)
    {
      DataTable dataTable = new DataTable();
      dataTable.Columns.Add("EmployeeID", typeof(Int32));
      dataTable.Columns.Add("LastName", typeof(String));
      dataTable.Columns.Add("Income", typeof(System.Decimal));

      dataTable.Rows.Add(1, "Davolio", 100);
      dataTable.Rows.Add(2, "Fuller", 100);
      dataTable.Rows.Add(3, "Leverling", 100);
      dataTable.Rows.Add(4, "Peacock", DBNull.Value);

      Debug.WriteLine("");
      Debug.WriteLine("Test1");
      Test1(dataTable);

      Debug.WriteLine("");
      Debug.WriteLine("Test2");
      Test2(dataTable);
    }

    private void Test1(DataTable dataTable)
    {
      Binding bdn = new Binding("Income");
      DataView view = dataTable.DefaultView;

      int ticks = Environment.TickCount;
      Decimal sumVal = 0;
      foreach (DataRowView rowView in view)
      {
        BindValueReader valueReader = new BindValueReader();
        valueReader.DataContext = rowView;
        valueReader.SetBinding(BindValueReader.ValueProperty, bdn);
        if (valueReader.Value != null)
          sumVal += (Decimal)valueReader.Value;
      }

      Debug.WriteLine("Sum of Income = " + sumVal.ToString());
      Debug.WriteLine("Ticks = " + (Environment.TickCount - ticks).ToString());
    }

    private void Test2(DataTable dataTable)
    {
      Binding bdn = new Binding("Income");

      BindValueReader valueReader = new BindValueReader();
      valueReader.SetBinding(BindValueReader.ValueProperty, bdn);

      DataView view = dataTable.DefaultView;
      Decimal sumVal = 0;
      int ticks = Environment.TickCount;

      foreach (DataRowView rowView in view)
      {
        valueReader.DataContext = rowView;
        if (valueReader.Value != null)
          sumVal += (Decimal)valueReader.Value;
      }

      Debug.WriteLine("Sum of Income = " + sumVal.ToString());
      Debug.WriteLine("Ticks = " + (Environment.TickCount - ticks).ToString());
    }
  }

  public class BindValueReader : FrameworkElement
  {
    public static readonly DependencyProperty ValueProperty =
      DependencyProperty.Register("Value",
                                  typeof(object),
                                  typeof(BindValueReader),
                                  new PropertyMetadata(null));

    public object Value
    {
      get { return GetValue(ValueProperty); }
      set { SetValue(ValueProperty, value); }
    }
  }

}

Debug Output:

Test1
System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=Income; DataItem='DataRowView' (HashCode=22689808); target element is 'BindValueReader' (Name=''); target property is 'Value' (type 'Object')
Sum of Income = 300
Ticks = 16

Test2
System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=Income; DataItem=null; target element is 'BindValueReader' (Name=''); target property is 'Value' (type 'Object')
Sum of Income = 0
Ticks = 0
System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=Income; DataItem='DataRowView' (HashCode=22689808); target element is 'BindValueReader' (Name=''); target property is 'Value' (type 'Object')

Answer 1

You're trying to sum the income property.

You can do that by working with the column in the datarowview you obtain rather than by using binding.

foreach (DataRowView rowView in view)
{
   var val = rowView["Income"];
   if (val != null)
   {
      sumVal += (Decimal)val;
   }
}

This is not a good way to work with WPF though and you should look into MVVM, define a class that implements inotifypropertychanged as a row viewmodel. Bind itemssource to an observablecollection of this. Then work with the bound collection and the named property.