Modifying a DataBound Collection - Page 3
       by kirupa  |  9 December 2007

In the previous page, we took inventory of our code and discussed what it does. Now, in this page, let's revisit our original problem and look at an easy way to solve it.

Our Original Problem
The last two pages gave you an overview of the application you downloaded and are about to modify. Now, let's revisit why you are reading this tutorial in the first place. We already have a data binding in place between our list of people and the listbox. What we want to do is use our textbox and Add button to add more items to our listbox.

The trick is, we want to do so while maintaining the existing data binding. We don't want to directly add the item to our listbox by using our listbox's Items collection, for that essentially overrides the data binding we have in place. Instead, we want to add the item to our existing data binding relationship maintained by our PeopleList object.

Now, if you looked at the code, you may be wondering where our PeopleList object actually lives. There is nowhere where you define a new object of type PeopleList and bind it to our listbox. The problem is that you are, more than likely, looking at the C# code. With WPF applications, you have code in your code-behind file, but you also have code in your XAML file. If you take a look at Window1.xaml, find the tag beginning with ObjectDataProvider inside your Window.Resources.

The line you will find should look like the following:

<ObjectDataProvider x:Key="PeopleListDS" d:IsDataSource="True" ObjectType="{x:Type CollectionsDataBinding:PeopleList}"/>

Notice that this particular line contains the information that is used by our listbox to bind to our PeopleList collection. What we need to do is gain access to this same PeopleList instance so that we can make modifications to the live data source used. Let's look at how to do just that.

Accessing our Data Source
As you saw in the preceding section, our PeopleList object is instantiated in the XAML file. Our event handler for the Add button, AddButton_Click is in the code-behind file written in C#. What we are going to do is access the data source defined in XAML using C# code.

Add the following lines of code in your AddButton_Click method directly above where you have the line NameInput.Text = String.Empty code:

ObjectDataProvider odp = this.FindResource("PeopleListDS") as ObjectDataProvider;
 

PeopleList people = odp.Data as PeopleList;

 

Person newPerson = new Person();

newPerson.PersonName = NameInput.Text;

 

people.Add(newPerson);

Your entire AddButton_Click event handler should look the following:

private void AddButton_Click(object sender, RoutedEventArgs e)

{

ObjectDataProvider odp = this.FindResource("PeopleListDS") as ObjectDataProvider;

PeopleList people = odp.Data as PeopleList;

 

Person newPerson = new Person();

newPerson.PersonName = NameInput.Text;

 

people.Add(newPerson);

 

NameInput.Text = String.Empty;

}

With your code copied and pasted, let's run this application. Press F5 or go to Debug | Start Debugging or simply hit the green Play button in your toolbar. In either case, your application will start to run. Once your application is displayed, type a name into your textbox and press Enter or click on the Add button:

[ a new name has been added! ]

For example, I typed in the name Marcus Fenix (from Gears of War fame) into my textbox and clicked on the Add button. Notice that the name I entered was added to my listbox! You basically solved the problem that was mentioned at the beginning of this tutorial and reiterated at the top of this page.

You are not home free just yet. In the next page, let's look at the code in detail and understand why the code worked.

Onwards to the next page!

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