Binding ObservableCollection to datagrid in MVVM - mvvm

Question

Im new to XAML, i am trying to bind ObservableCollection to datagrid in MVVM.
I want to get notified when CollectionChanged. But its throwing null exception.
Please let me know when im going wrong. Thanks in advance.
The following is Code behind for viewModel:
public class MainwindowViewModel : INotifyPropertyChanged
{
MyObject myObj;
ObservableCollection<MyObject> _ocObj;
public MainwindowViewModel()
{
_ocObj = new ObservableCollection<MyObject>();
myObj = new MyObject();
myObj.ID = 0;
myObj.Name = "Name";
_ocObj.Add(myObj);
_ocObj.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler(_ocMyobj_CollectionChanged);
}
void _ocMyobj_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
{
System.Windows.MessageBox.Show("propeties changed # " + e.NewStartingIndex.ToString()
+ " old items starting # " + e.OldStartingIndex + " olditems count " + e.OldItems.Count.ToString()
+ " action " + e.Action.ToString());
}
public ObservableCollection<MyObject> ocObj
{
get { return _ocObj; }
set
{
PropertyChanged(this, new PropertyChangedEventArgs("ocMyobj"));
}
}
public string Name
{
get { return myObj.Name; }
set
{
if (value !=null)
{
myObj.Name = value;
PropertyChanged(this, new PropertyChangedEventArgs("Name"));
}
}
}
public int ID
{
get { return myObj.ID; }
set
{
if (myObj.ID != value)
{
myObj.ID = value;
PropertyChanged(this, new PropertyChangedEventArgs("ID"));
}
}
}
#region INotifyPropertyChanged Members
public event PropertyChangedEventHandler PropertyChanged;
#endregion
}
public class MyObject
{
public string Name { get; set; }
public int ID { get; set; }
}
Below is XAML:
<Window.Resources>
<vm:MainwindowViewModel x:Key="someObj"/>
</Window.Resources>
<DataGrid ItemsSource="{Binding ocObj}" DataContext="{Binding Source={StaticResource someObj}}" AutoGenerateColumns="True" />

Answer 1

Take a look at the documentation on the NotifyCollectionChangedEventArgs class. Note that the OldItems object only "Gets the list of items affected by a Replace, Remove, or Move action." What this means is that for other actions, OldItems will be null.
Therefore, if an Add action is performed against your ObservableCollection, OldItems is null (and valid). Just perform a check for it in your code, such as:
System.Windows.MessageBox.Show("propeties changed # " + e.NewStartingIndex.ToString()
+ " old items starting # " + e.OldStartingIndex + " olditems count " +
(e.OldItems == null ? "0" : e.OldItems.Count.ToString())
+ " action " + e.Action.ToString());