WPF datagrid edit row not working

I want to set a cell in my WPF DataGrid on a 开发者_开发技巧button click.

I fill a WPF DataGrid like this:

myDataGrid.ItemsSource = GetMyList();

The DataGrid is set to autoGenerateColumns. I get my list using a mySql select.

The objects in my list implement the INotifyPropertyChanged interface.

On my button click I do this:

MyObject o = (MyObject)myDataGrid.SelectedItem;
o.Checkin = DateTime.Now; //set date on button click is what i want

The data is set but the DataGrid doesn't update its view. Why?

Edited: I implement the INotifyPropertyChanged interface like this:

private void NotifyPropertyChanged(String info)   {
    if (PropertyChanged != null)    {
        PropertyChanged(this, new PropertyChangedEventArgs(info));
    }
}

public DateTime Checkin    {
    get {return this.checkin;}       
    set  {
            this.checkin= value;
            NotifyPropertyChanged("Checkin");
        }
    }
}

this did the trick:

   myDataGrid.Items.Refresh();

what happens if you dont use autogeneratecolumns and explicitly set the binding in your columnsdefinition to TwoWay?

  <DataGridTextColumn Header="Checkin " >
            <DataGridTextColumn.Binding>
                <Binding Path="Checkin " Mode="TwoWay">
                </Binding>
            </DataGridTextColumn.Binding>
    </DataGridTextColumn>