Binding to Combo Boxes in VS .NET

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Not all combo boxes are used for data entry Often you need to give the users a set of choices that are handled programmatically For example, suppose you want to allow the users to choose how to lter a grid or which item to view or edit You may want to provide a list of reports they could select, or operations they could perform When you provide a combo box for programmatic use, you do not assign the user s selection to a business object property Instead, you write code to react based on the user s selection To bind to a combo box for programmatic use: 1 De ne the data structure for the list of valid values in your code Because the binding in Visual Studio allows you to bind to many different types of structures, you have many choices You can use anything from an array to a DataSet to a generic collection, such as a List 2 Set up the binding This step involves setting up the combo box for binding and adding the one line of code that associates the data structure de ned in step 1 with the DataSource property of the BindingSource When binding to a combo box for programmatic use, two ComboBox control properties need to be set: DisplayMember de nes the property containing the data you want displayed in the combo box This is often a name or description For the Product combo box example, this is the ProductName property to display the product names in the list ValueMember de nes the property containing the data you need as the value of the combo box selection in your code For the Product combo box example, you set this to the ProductID property so that your code has access to the product s ID
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NOTE: Do not set the SelectedValue property If you do, the combo box behaves oddly because it is attempting to retain your selection
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7 BINDING THE USER INTERFACE TO THE BUSINESS OBJECTS
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3 Add code to the appropriate event to programmatically respond to the user s selection
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You normally use the SelectedValueChanged event of the ComboBox control for this When the user selects a new value, your code responds accordingly In the Product combo box example, the code retrieves a different product for reviewing or editing These steps are discussed in detail in the next sections
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Selecting the Data Structure
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The rst step to binding a combo box is to select the data structure for storing the list of choices for display in the combo box The data structure you choose depends on where the information for the list resides, the number of list entries, and your personal preferences The built-in generic List class is a good choice if you are displaying a list of choices that are hard-coded in your application, like a sort order (Ascending, Descending, None) or display options (single document, multiple document, default) Create a generic List in your code, add the choices to the List, and bind the ComboBox control as follows:
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Dim sortTypeList As New List(Of String) sortTypeListAdd("Ascending") sortTypeListAdd("Descending") sortTypeListAdd("None") SortTypeComboBoxDataSource = sortTypeList
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This code binds the combo box directly to the List without using the Data Sources window or a BindingSource This straight-forward binding is great when binding to simple lists of choices Another data structure option is to use a collection class Build a business object class that de nes a choice, and then build a collection class to manage the set of choices, as described in 6 The runtime creates an instance of the business object class for every choice in the list A collection class is a good data structure to use if you have a reasonably sized list of items that are stored in a database or some other source A DataTable, like the one used when binding to a data entry combo box, is also a good data structure option Build a class that contains the property names to be associated with the ComboBox control s DisplayMember and ValueMember Add the code to populate the DataTable with the list of choices The runtime then populates the DataTable and displays the set of
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