visual basic .net barcode generator Configuring the ComboBox Control in Visual C#.NET

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Configuring the ComboBox Control
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We are going to use the ComboBox control to filter the data displayed in the DataGrid by the value of the City field. We start by adding the list of cities that will be available in the drop-down list. Select the ComboBox control in the design surface, switch to the Properties window, and click the ellipsis (...) button belonging to the Items property. This will open the Collection Editor dialog box again. Click the Add button to create a new item in the list and change the value of the Content property to All. Repeat this process so that you have items for the following city names as well:
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London Redmond Seattle Tacoma
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Click OK to dismiss the Collection Editor when you have added all the items. Return to the Properties window for the ComboBox control, and change the value of the SelectedIndex property to 0. This has the effect of selecting the first item (All) by default when the display is created. The design surface will update so that All is shown as the selected value for the ComboBox, as shown in Figure 3523.
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Figure 35-23. Configuring the ComboBox control
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Wiring the ComboBox Control
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All that remains is for us to filter the displayed data based on the selection in the ComboBox. Select the ComboBox, and select the Events view in the Properties window. Double-click in the value area for the SelectionChanged property to create a new handler method. Add the code statements as shown in Listing 35-3. Listing 35-3. Wiring the ComboBox using System.Windows.Controls; namespace DataApp { public partial class MainPage : UserControl { public MainPage() {
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CHAPTER 35 SILVERLIGHT
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InitializeComponent(); } private void comboBox1 SelectionChanged(object sender, SelectionChangedEventArgs e) { if (comboBox1 != null) { // get the selected city from the ComboBox string selectedCity = ((ComboBoxItem)comboBox1.SelectedItem).Content.ToString(); // remove any filters that are in place employeeDomainDataSource.FilterDescriptors.Clear(); // add a new filter if the user has selected a specific city if (selectedCity != "All") { employeeDomainDataSource.FilterDescriptors.Add( new FilterDescriptor( "City", FilterOperator.IsEqualTo, selectedCity)); } } } } } There is an oddity when handling the SelectionChanged event, which is that it is triggered when the initial selection is made during the interface layout, but before the ComboBox object is assigned to the instance variable we need to refer to. This is why I have started the handler code by checking to see whether the ComboBox variable is null, like this:
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if (comboBox1 != null) {
If it is null, then we assume that the user has not made a selection and ignore the event. If the user has made a selection, then I next read the selected value from the ComboBox, like this:
string selectedCity = ((ComboBoxItem)comboBox1.SelectedItem).Content.ToString();
The ComboBox control can contain a wide range of item types, which makes the process of reading the value slightly indirect. The SelectedItem property returns a ComboBoxItem. I read the Content property to get the value assigned to this item and then call the ToString method to convert it to a string value. The DomainDataSource object supports a system of filters, the collection of which is accessed using the FilterDescriptors property. I call the Clear method on this collection to remove any previous filters that might have been applied. If the user has switched the filter from London to Seattle, I want to remove the London filter before I apply the Seattle filter. No data will be displayed if I don t. Here is the statement that clears the filter collection:
employeeDomainDataSource.FilterDescriptors.Clear();
The remaining statements check to see whether a filter is needed and, if so, create one: if (selectedCity != "All") {
CHAPTER 35 SILVERLIGHT
employeeDomainDataSource.FilterDescriptors.Add( new FilterDescriptor( "City", FilterOperator.IsEqualTo, selectedCity)); } I don t need to apply a filter if the user has selected the All item in the ComboBox. If the user has selected any other item, then I create a new FilterDescriptor object and add it to the DomainDataSource by calling the Add method on the result of the FilterDescriptors property. The FilterDescriptor constructor takes three parameters the name of the property that you want to filter on, a value from the FilterOperator enumeration that specifies what kind of comparison to make (IsEqualTo, IsNotEqualTo, Contains, and so on), and the value to compare to. My filter selects data rows that have a City value that is equal to the name the user selected in the ComboBox.
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