Working with Data Sets in VS .NET

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62 Working with Data Sets
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Figure 65: Three Ways to Designate Which Row to Display
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is automatically reflected in the single-item controls In our example, the binding is done in the form s Load event handler, as shown here:
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// Bind the ComboBox with the Product names comboProductIDsDisplayMember = strPKDesc; comboProductIDsValueMember = strPKName; comboProductIDsDataSource = dtabProducts; comboProductIDsSelectedIndex = 0; // Bind the DataTable's columns to the text boxes textProductIDDataBindingsAdd ("Text", dtabProducts, strPKName); textProductNameDataBindingsAdd ("Text", dtabProducts, strPKDesc); textCategoryNameDataBindingsAdd ("Text", dtabProducts, strFKDesc);
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When the user taps on an entry in the combo box, that row is displayed in the text boxes If the user enters new values in the text boxes, those values are updated to the data set as soon as the user selects a new row; and the validation events for the text box fire before the row is updated What more could we want When writing the code to do data binding, set the DisplayMember property prior to setting the DataSource property Setting the DataSource property causes things to happen, some of them visible to the user Therefore, you want everything in place before setting the DataSource property To
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illustrate this, you can take the sample code shown on the previous page and reposition the two calls so that the DataSource property is set prior to the DisplayMember property When you run the code you may notice a slight flicker in the combo box Indexing Indexing is easy to implement but not as easy as matching binding And although indexing is easy, it is not obvious When the user positions the scroll bar to a value of n, the application knows that it needs to make row n the current row of the data table Unfortunately, data tables do not have a current row property because the concept of a current row is meaningful only within data binding To maintain a current row context when not data-bound would be wasted overhead Because data binding involves controls and because controls reside within forms, the Form class assumes the responsibility for managing the bindings and for maintaining the concept of the current row When the first binding in the previous code excerpt that involved the dtabProducts data table executed, the form created an instance of the BindingContext class for managing dtabProducts bindings As the subsequent binding code executed, that object was updated with information about the additional bindings When the user tapped on the nth entry in the combo box, the form reacted to the combo box s SelectedIndexChanged event by noting that the combo box was bound to dtabProducts and that the text boxes were also bound to dtabProducts So, the form notified the text boxes to obtain their Text property values from row n of the data table, and it updated the binding context object for dtabProducts to have a Position property value of n To control the current row of a bound data table and thus control which row is displayed in the text boxes, you need to tap into the form s binding management capability Specifically, you need to obtain the CurrencyManager object for the data table (or data view) and set its Position property s value to n The CurrencyManager object for a data object is contained in the default collection within a form s BindingContext object In our application, this means handling the scroll bar s ValueChanged event as shown in the following line of code (assuming the scroll bar is named hsbRows and its minimum and maximum values are 0 and the number of rows in the data object minus 1, respectively):
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thisBindingContext[dtabProducts]Position = hsbRowsValue;
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62 Working with Data Sets
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This causes the text boxes to update their Text property values with the values from the row of the dtabProducts table indicated by hsbRowsValue It seems like we are saying that indexing implies assigning controls according to the current row Instead, you might wonder what s wrong with binding the text boxes to dtabProductsDefaultView, rather than to the data table itself, and then reacting to the scroll bar s ValueChanged event by setting the view s Filter property with the primary key of the nth row, as shown here:
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dtabProductsDefaultViewRowFilter = "ProductID = " + dtabProductsRows[hsbRowsValue]["ProductID"];
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Now the text boxes are bound to a view that contains only one row, which is the row they display, and the concept of current row becomes meaningless You can do it this way, and it works fine until you try to update the fields by entering new values into the text boxes When you enter data into the text boxes and then reposition the scroll bar thumb, two things go wrong First, tapping in the scroll bar does not cause the text box to lose focus, and thus the validation events do not fire Second, you do not move from one row to another row within the bound view; instead, you replace the contents of the bound view with new contents, and thus the values in the underlying data table are not updated So, although indexing does not necessarily imply assigning controls by current row, we recommend doing it that way whenever you choose to use indexing And as you just saw, you can implement that choice in one line of code Search Key The third method to use when designating the desired row has the user enter a search value, which fits nicely with the indexing option because both are value-based rather than position-based techniques The single-item controls can be bound to the data table s default view, as shown in the following code:
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// Bind the DataTable's columns to the text boxes textProductIDDataBindingsAdd ("Text", dtabProducts, strPKName); textProductNameDataBindingsAdd ("Text", dtabProducts, strPKDesc); textCategoryNameDataBindingsAdd ("Text", dtabProducts, strFKDesc);
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