WPF

public class MyItem : INotifyPropertyChanged
{
  private bool _selected;
  private string _name;
  private event PropertyChangedEventHandler _changed;
  public event PropertyChangedEventHandler Changed
  {
    add { _changed += value; }
    remove { _changed -= value; }
  }

  public bool Selected
  {
    get { return _selected; }
    set
    {
      if (_selected != value)
      {
        _selected = value;
        OnChanged("Selected");
      }
    }
  }
  // Name code ommitted for brevity...
}
public class MyViewModel
{
  private ObservableCollection<MyItem> _items = new ObservableCollection<MyItem>();

  public void HandleCheckAll
  {
    foreach (MyItem item in _items)
    {
      item.Selected = true;
    }
  }

  public ObservableCollection<MyItem> Items
  {
    get { return _items; }
  }

  // Some code to populate the items would need to go here....
}

<toolkit:DataGridTextColumn  Header="Name" Width="100"  >
    <toolkit:DataGridTextColumn.Binding  >
        <Binding Path="Name" >
            <Binding.ValidationRules>
                <local:NameRule ValidationStep="UpdatedValue"/>
            </Binding.ValidationRules>
        </Binding>
    </toolkit:DataGridTextColumn.Binding>
</toolkit:DataGridTextColumn>

Public Class UserViewModel
    Implements INotifyPropertyChanged
    Implements IDataErrorInfo

    Private _user As ATUser
    Private _containerCollection As AUSObservableCollection(Of UserViewModel)

    Public Sub New(ByRef User As ATUser, _
                   ByRef ContainerCollection As AUSObservableCollection(Of UserViewModel))
        _user = User
        _containerCollection = ContainerCollection
        _username = _user.Username
    End Sub

    'lines removed
    Default Public ReadOnly Property Item(ByVal propertyName As String) As String Implements IDataErrorInfo.Item
        Get
            Select Case propertyName
                Case "Username"
                'logic to determine if Username is valid, which involves searching through _containerCollection 
            End Select
        End Get
    End Property
End Class

Public Class ATBusinessInfoHelper

    Private Shared _userContainer As AUSObservableCollection(Of ATUser)

    Public Shared Property UserContainer() As AUSObservableCollection(Of ATUser)
        Get
            If _userContainer Is Nothing Then
                Throw New AUSException("UserContainer has not been defined")
            Else
                Return _userContainer
            End If
        End Get
        Set(ByVal value As AUSObservableCollection(Of ATUser))
            _userContainer = value
        End Set
    End Property
End Class