DataGrid bound to List not letting user add new row

Question

I know that this question is duplicate of several others because I have been studying those "several others." And I have been unable to figure out where I'm messing up.

This post is specifically about:

C#
WPF
DataGrid
List<>
Binding DataGrid to the List<>

When I run my code, DataGrid allows me to change and delete rows. DataGrid does not allow me to add a row. Why?

Xaml:

Code Behind:

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

namespace myNamespace
{
    /// 
    /// Interaction logic for MainWindow.xaml
    /// 
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();

            init_item_list();
            dataGrid.ItemsSource = ItemList;
        }


        public List ItemList = new List();

        public void init_item_list()
        {
            ItemList.Add(new DataItem(1, "one", "i"));
            ItemList.Add(new DataItem(2, "two", "ii"));
            ItemList.Add(new DataItem(3, "three", "ii"));
        }
        public class DataItem
        {
            public DataItem(int id, string val_1, string val_2)
            {
                value_id = id;
                value_1 = val_1;
                value_2 = val_2;
            }
            private int _value_id;
            public int value_id
            {
                get { return _value_id; }
                set { _value_id = value; }
            }

            private string _value_1;
            public string value_1
            {
                get { return _value_1; }
                set { _value_1 = value; }
            }

            private string _value_2;
            public string value_2
            {
                get { return _value_2; }
                set { _value_2 = value; }
            }


        }

    }
}

I believe I saw rows added from the user interface when DataGrid is bound to a List. However, I don't see it working with my code here.

Efraim Newman commented that could be the problem is using List. I have seen mention in other posts that ObservableCollection works better than List for DataGrid binding.

In the code behind, I changed

            public List ItemList = new List();

to

public ObservableCollection ItemList = new ObservableCollection();

Before the change (using List), the running DataGrid looked like this:

After the change (using ObservableCollection), the running DataGrid looked like this:

In either case, there is no blank row by which to add a new record.

I do not see any difference using ObservableCollection instead of List. DataGrid still does not allow me to add a record.

I think the advantage of using ObservableCollection is that ObservableCollection notifies (causes DataGrid) to update whenever there is a change in ObservableCollection. That does not seem to be the problem here.

Here, the problem is that DataGrid is not accepting any new entries.

In Xaml I tried explicitly setting CanUserAddRows to True and even to False.

The Answer

@mm8 found my problem: I needed to add a default, parameterless constructor. I added:

            public DataItem() { }

The Xaml remains as it was originally. Note I am now binding to List.

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

namespace myNamespace
{
/// 
/// Interaction logic for MainWindow.xaml
/// 
public partial class MainWindow : Window
{
    public MainWindow()
    {
    InitializeComponent();

    init_item_list();
    dataGrid.ItemsSource = _itemList;
    }


    private List _itemList = new List();
    public List ItemList
    {
    get { return _itemList; }
    set { _itemList = value; }
    }

    public void init_item_list()
    {
    _itemList.Add(new DataItem(1, "one", "i"));
    _itemList.Add(new DataItem(2, "two", "ii"));
    _itemList.Add(new DataItem(3, "three", "ii"));
    }
    public class DataItem
    {
    public DataItem() { }
    public DataItem(int id, string val_1, string val_2)
    {
        value_id = id;
        value_1 = val_1;
        value_2 = val_2;
    }
    private int _value_id;
    public int value_id
    {
        get { return _value_id; }
        set { _value_id = value; }
    }

    private string _value_1;
    public string value_1
    {
        get { return _value_1; }
        set { _value_1 = value; }
    }

    private string _value_2;
    public string value_2
    {
        get { return _value_2; }
        set { _value_2 = value; }
    }

}

I think an example where you cannot use List is where you will change List directly and need DataGrid to show the changes.

I have tested this code using both List and ObservableCollection. It now works from adding the default, parameterless constructor.

mm8 · Accepted Answer

A List doesn't implement the INotifyCollectionChanged interface that is required for the view to be able to subscribe to items being added or removed and refresh the control automatically.

The only built-in class that implements this interface is ObservableCollection. If you change the type of ItemList, it should work as expected:

public ObservableCollection ItemList = new ObservableCollection();

You should also consider either making ItemList a public property or rename it to _itemList and make it a private field:

private readonly ObservableCollection _itemList = new ObservableCollection();

For the DataGrid to display a blank row that lets you add an item to the source collection, your DataItem class must define a default parameterless constructor:

public DataItem() { }

DataGrid bound to List not letting user add new row

Xaml:

Code Behind:

The Answer

Answers (2)

Related Questions