If you want full programmatic control over validation, or need to perform complex validation checks, you should use the validation events built into most Windows Forms controls. Each control that accepts free-form user input has a Validating event that will occur whenever the control requires data validation. In the Validating event-handling method, you can validate user input in several ways. For example, if you have a text box that must contain a postal code, you can perform the validation in the following ways:
If the postal code must belong to a specific group of zip codes, you can perform a string comparison on the input to validate the data entered by the user. For example, if the postal code must be in the set {10001, 10002, 10003}, then you can use a string comparison to validate the data.
If the postal code must be in a specific form you can use regular expressions to validate the data entered by the user. For example, to validate the form ##### or #####-####, you can use the regular expression ^(\d{5})(-\d{4})?$. To validate the form A#A #A#, you can use the regular expression [A-Z]\d[A-Z] \d[A-Z]\d. For more information about regular expressions, see .NET Framework Regular Expressions and Regular Expression Examples.
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 is not valid, you can cancel the Validating event by setting this object's Cancel property to true. If you do not 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 e-mail address in a TextBox, see Validating.
Data Binding and Event-Driven Validation
Validation is very 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 does not 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 will not be synchronized with the data source.
.gif) Important Note: |
|---|
If you have custom validation that takes place after the
Validating event, it will not 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 Data Source Update Mode property (under (Databindings)\(Advanced)) from OnValidation to Never, and add Control.DataBindings["<YOURFIELD>"].WriteValue() to your validation code.
|
Implicit and Explicit Validation
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.
Implicit Validation
The implicit validation approach validates data as the user enters it. You can validate the data as the data is entered in a control by reading the keys as they are pressed, or more commonly whenever the user takes the input focus away from one control and moves to the next. This approach is useful when you want to give the user immediate feedback about the data as they are working.
If you want to use implicit validation for a control, you must set that control's AutoValidate property to true. If you cancel the Validating event, the behavior of the control will be determined by what value that 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 input. If you assigned EnableAllowFocusChange, the Validated event will not 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. To validate your controls, you will have to use explicit validation.
Explicit Validation
The explicit validation approach validates data at one time. You can validate the data in response to a user action, such as clicking a Save button or a Next link. When the user action occurs, you can trigger explicit validation in one of the following ways:
Call Validate to validate the last control to have lost focus.
Call ValidateChildren to validate all child controls in a form or container control.
Call a custom method to validate the data in the controls manually.
Default Implicit Validation Behavior for Windows Forms Controls
Different Windows Forms controls have different defaults for their AutoValidate property. The following table shows the most common controls and their defaults.