WPF DataGrid SelectedItem

Question

I have a DataGrid a user can add items to by entering data in the last row. I also have a button that deletes the currently selected item. But when the last (empty, for adding new items) row is selected, whatever was the last selected item remains in SelectedItem. So if I open the window, select the last row, and press the delete button, it will delete the first row, as it is selected by default, and selecting the last row did not change SelectedItem. Any good way to deal with this?

To clarify: SelectedItem="{Binding X}"

X in the ViewModel does not change when the last row is selected (the setter isn't invoked at all). I'm not sure whether the SelectedItem property itself changes, but I would assume it doesn't.

There is also an exception when I select the last row (red border), but when I click it again to start entering data, the red border disappers. Not sure if these two are related.

Answer 1

Run the following example and you'll see why it doesn't work.

XAML:

<Window x:Class="DataGridTest.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <DockPanel>
        <TextBlock DockPanel.Dock="Bottom" Text="{Binding SelectedItem, ElementName=dataGrid}"/>
        <TextBlock DockPanel.Dock="Bottom" Text="{Binding SelectedItem}"/>
        <DataGrid x:Name="dataGrid" ItemsSource="{Binding Items}" SelectedItem="{Binding SelectedItem}" CanUserAddRows="True" CanUserDeleteRows="True" AutoGenerateColumns="False">
            <DataGrid.Columns>
                <DataGridTextColumn Header="First Name" Binding="{Binding FirstName}"/>
                <DataGridTextColumn Header="Last Name" Binding="{Binding FirstName}"/>
            </DataGrid.Columns>
        </DataGrid>
    </DockPanel>
</Window>

Code-behind:

namespace DataGridTest
{
    using System.Collections.Generic;
    using System.Collections.ObjectModel;
    using System.ComponentModel;
    using System.Windows;

    public partial class MainWindow : Window, INotifyPropertyChanged
    {
        private readonly ICollection<Person> items;
        private Person selectedItem;

        public MainWindow()
        {
            InitializeComponent();

            this.items = new ObservableCollection<Person>();
            this.items.Add(new Person
                {
                    FirstName = "Kent",
                    LastName = "Boogaart"
                });
            this.items.Add(new Person
            {
                FirstName = "Tempany",
                LastName = "Boogaart"
            });

            this.DataContext = this;
        }

        public ICollection<Person> Items
        {
            get { return this.items; }
        }

        public Person SelectedItem
        {
            get { return this.selectedItem; }
            set
            {
                this.selectedItem = value;
                this.OnPropertyChanged("SelectedItem");
            }
        }

        private void OnPropertyChanged(string propertyName)
        {
            var handler = this.PropertyChanged;

            if (handler != null)
            {
                handler(this, new PropertyChangedEventArgs(propertyName));
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;
    }

    public class Person
    {
        public string FirstName
        {
            get;
            set;
        }

        public string LastName
        {
            get;
            set;
        }

        public override string ToString()
        {
            return FirstName + " " + LastName;
        }
    }
}

As you can see when running, selecting the "new" row causes a sentinel value to be set as the selected item in the DataGrid. However, WPF is unable to convert that sentinel item to a Person, so the SelectedItem binding fails to convert.

To fix this, you could put a converter on your binding that detects the sentinel and returns null when detected. Here's a converter that does so:

namespace DataGridTest
{
    using System;
    using System.Windows.Data;

    public sealed class SentinelConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            return value;
        }

        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            if (value.Equals(CollectionView.NewItemPlaceholder))
            {
                return null;
            }
            return value;
        }
    }
}

As you can see, it is an unfortunate necessity to test against the ToString() value of the sentinel, because it is an internal type. You could alternatively (or in addition) check that GetType().Name is NamedObject.

Answer 2

It sounds like you forgot to set the binding Mode and the default is set to OneWay. Which means whatever changes made in your View will not propagate back to your view model.

And always make sure you have the right datacontext.

Hope that helps.

Answer 3

Difficult to say without code, but I would look at the following.

Ensure that whenever an item is deleted and it is also the selected item, set the selected item bound to property in your ViewModel to null. You'll need to ensure that your SelectedItem bound to property isn't bound oneway.