These additional validity constraints may involve cross-referencing supplied data with a known look-up table or directory information service such as LDAP.For example, an experienced user may enter a well-formed string that matches the specification for a valid e-mail address, as defined in RFC 5322 but that well-formed string might not actually correspond to a resolvable domain connected to an active e-mail account. NET Framework 2.0, we recommend that you use the Xml Reader Settings class and the Xml Reader Sets the Xml Resolver used for resolving external document type definition (DTD) and schema location references.
For example, many database systems allow the specification of the following l (plus, minus, and parentheses).
A judgement as to whether data is valid is made possible by the validation program, but it cannot ensure complete accuracy.
This can only be achieved through the use of all the clerical and computer controls built into the system at the design stage.
The check step uses one or more computational rules (see section below) to determine if the data is valid.
The Post-validation action sends feedback to help enforce validation.