Assembly: Microsoft.SharePoint (in microsoft.sharepoint.dll)
In a custom type-specific class, override the GetValidatedString method to define the following kinds of logic:
Data-validation, for example, to throw an exception in server-side code if the field is required.
Pre-update data processing, for example, to filter out bad data but not fail validation.
Serialization, for example, to convert a field value object into string format.