Initializes a new instance of the RuleObjectExistsAttribute class.
A string value representing the criteria that must be satisfied by the target type objects.
The common rules of writing a criteria are described in the Ways to Build Criteria topic. In addition, you can use the following construction:
PropertyName = '@PropertyName' (the operator is optional). @PropertyName represents a parameter that is set to the value of the PropertyName property of the currently validated object, when the rule is checked.