For more information about regular expressions, see. If the postal code must be a valid United States Zip code, you could call a Zip code Web service to validate the data entered by the user. The Validating event is supplied an object of type CancelEventArgs.
If you determine that the control's data isn't valid, cancel the Validating event by setting this object's Cancel property to true. If you don't set the Cancel property, Windows Forms will assume that validation succeeded for that control and raise the Validated event. For a code example that validates an email address in a TextBox , see the Validating event reference. Validation is useful when you have bound your controls to a data source, such as a database table.
By using validation, you can make sure that your control's data satisfies the format required by the data source, and that it doesn't contain any special characters such as quotation marks and back slashes that might be unsafe.
When you use data binding, the data in your control is synchronized with the data source during execution of the Validating event. If you cancel the Validating event, the data won't be synchronized with the data source. If you have custom validation that takes place after the Validating event, it won't affect the data binding.
For example, if you have code in a Validated event that attempts to cancel the data binding, the data binding will still occur. In this case, to perform validation in the Validated event, change the control's Binding. Never , and add your-control. WriteValue to your validation code. So when does a control's data get validated? This is up to you, the developer.
You can use either implicit or explicit validation, depending on the needs of your application. The implicit validation approach validates data as the user enters it. Validate the data by reading the keys as they're pressed, or more commonly whenever the user takes the input focus away from the control.
This approach is useful when you want to give the user immediate feedback about the data as they're working. If you cancel the Validating event, the behavior of the control will be determined by what value you assigned to AutoValidate. If you assigned EnablePreventFocusChange , canceling the event will cause the Validated event not to occur. Input focus will remain on the current control until the user changes the data to a valid format. If you assigned EnableAllowFocusChange , the Validated event won't occur when you cancel the event, but focus will still change to the next control.
Assigning Disable to the AutoValidate property prevents implicit validation altogether. Thus in masked mode the entered values always match the edit masks. In some cases, however, you may want to provide manual validation control limit the range of valid values, implement a conditional validation mechanism, etc. For this purpose the RepositoryItem.
Validating event should be handled. This event lets you check the value entered before it is accepted. The RepositoryItem. Validating event is raised when an end-user attempts to accept the edited value.
This occurs in the following cases:. When using masks in editors the entered value is automatically checked against the mask first. In specific cases the value may fail validation. Validating event will be fired as a result and its Cancel parameter is set to true by default. By default when the editor fails validation an error icon with a tooltip is displayed. The editor remains focused thus forcing end-users to correct the entered value:. To implement a custom response to invalid values being entered the BaseEdit.
InvalidValue event should be handled. For instance, you can suppress the default behavior of an error icon and tooltip being displayed and display a custom message instead or revert to the old value, etc. ExceptionMode parameter:. Displays a tooltip with an error description specified by the ExceptionEventArgs. ErrorText parameter. In standalone editors, because of the.
In container controls, the Ignore value works as expected. Throws an exception specified by the ExceptionEventArgs. Exception property. In the following example a custom validation algorithm is implemented via the RepositoryItem. Validating event. Assume that a date editor should accept values that belong to the current month only. All other values are invalid. You can also show editor errors using the BaseEdit. ErrorText property. If an error occurs, set this property to a string that describes the error.
The editor will display an error icon. Pointing to that icon displays a hint with the error message specified. Note: you need to reset the BaseEdit. ErrorText property if validation is successful. This will hide the error icon. Become a more social person.
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