DataGridView Validation Events in VS .NET

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DataGridView Validation Events
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The DataGridView control is a Windows Forms control derived from theControl base class, and it is a complex data container The DataGridView control lets you handle validation at several levels The grid itself is a control, and thus raises Validating and Validated events when the focus shifts away from the grid However, usually you will want to handle validating a little closer to the data that is being validated, either at the individual row or cell level To allow this, the DataGridView fires RowValidating and RowValidated events each time the selected row changes within the grid Likewise, as the focus shifts from cell to cell within the grid, CellValidating and CellValidated events fire as well These events follow the same pattern as the control validating events, letting you cancel validation by setting their event argument Cancel property to true In the case of the RowValidating event, the event argument type is DataGridViewCellCancelEventArgs; for CellValidating, the event argument type is DataGridViewCellValidatingEventArgs Both of these types give you access to the current RowIndex and ColumnIndex, and they have aCancel property that can be set to TRue to cancel validation The DataGridView control is designed to keep the focus on the current cell if validation fails As you may remember from earlier in the book, each time you shift focus in a data-bound grid, a CellParsing event fires for the cell you are leaving, and a CellFormatting event fires for the cell you have moved to These events let you modify the displayed data as it goes out to and comes in from the data source, respectively With respect to validation, you should be aware that the CellValidating event fires before the CellParsing event So the validation logic you apply in a handler for theCellValidating event should validate against the display patterns for the cell, which don't necessarily map directly to the storage patterns for the corresponding data member in the data source If you are doing conversions or formatting for display purposes, and want to validate the data before it gets pushed back into the underlying data source but after the parsing process has occurred, you will want to call that logic in the CellParsing event handler, not the CellValidating event handler
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The ContainerControl class (which Form and UserControl derive from) defines a Validate method that will validate the currently focused control in the container, and then will walk up the control hierarchy to each of its ancestors in the control tree, validating each of them as well For a typical dialog-style form, each control is a child of the form, so the only ancestor of every control on the form is the form itself The Form class does nothing in response to validation itself, because it doesn't directly contain input data If you have a form that contains other container controls, such as user controls or split containers, then the container control will be the immediate ancestor of any controls it contains, and the form will be the ancestor for the container control The Validate method was often used in NET 11 applications to let you programmatically check whether all the controls on the form were valid However, because the Validate method only checks the currently focused control and its ancestors, you had to check all controls on the form to iterate through the Controls collection on the form, set the focus to each one, and then call Validate This approach was tedious and problematic, so a better approach was needed You can still call Validate on the form or on a user control to programmatically invoke validation on the currently focused control if needed, but you will probably want to use the new ValidateChildren method more often If you choose to use the Validate method, it returns TRue if validation is successful, which again is determined by whether the focused control, or any of its ancestor controls, sets the Cancel property on the Validating event argument to true If the focused control or any control up the control hierarchy from that control votes no by setting Cancel to TRue, then the Validate method will return false, and your code should take appropriate measures to make users aware of the problem, and you will usually want to prevent them from moving on until the problem is corrected
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