Databases Reference
In-Depth Information
NOTE
Validation attributes applied to properties and fields are evaluated before the attributes
applied to the class. However, you should not rely on the order of validation within
these two groups. The .NET Framework does not guarantee that the order of properties
or attributes returned by the reflection APIs will match the order of their appearance in
the source code. Even though, in many cases, the order of validation appears to match
the order of attributes, there will be exceptions. If the order of validation rules is impor-
tant, instead of attributes, you need to rely on coded validation discussed in the
Imperative Validation
section of this chapter.
If you want to validate an object without throwing a
ValidationException
, you can call
the
TryValidateObject
method of the
Validator
class instead:
bool TryValidateObject(object instance, ValidationContext validationContext,
ICollection<ValidationResult> validationResults
, bool validateAllProperties)
This method returns a
Boolean
value that indicates whether or not the object is valid and
populates the collection you supply in the
validationResults
parameter with a separate
ValidationResult
object for each error detected.
Validation Error Messages
In addition to performing the validation itself, the validation attributes are responsible for
generating error messages if the validation fails. Each concrete validation attribute, such as
RequiredAttribute
, discussed so far, provides a default error message. For instance, with
the following definition of the ProductName property, the
RequiredAttribute
generates
an error message “The ProductName field is required.”
[Required]
public string ProductName { get; set; }
Obviously, this error message is very technical and might not be appropriate for business
users. You can replace the internal name of the field in the error message with its human-
readable form by applying the
DisplayAttribute
as shown here:
[Required]
[Display(Name = “Product Name”)]
public string ProductName { get; set; }
With this updated definition, the
RequiredAttribute
generates the error message as “The
Product Name field is required,” which in many instances will be acceptable for display.
However, if this is still not enough, you can completely replace the error message by speci-
fying the
ErrorMessage
property in the validation attribute explicitly:
