Implementing Entity Validation - ASP.NET Dynamic Data - page 259

}
In this example, a custom validation method called ValidateOrderStatus is associated
with the OrderStatus property in the metadata class. The validation method first retrieves
the ObjectStateEntry instance that contains the Order entity being validated from the
Items dictionary of the ValidationContext object. It then checks the current State of the
entity and only performs the validation if the entity is Modified . Because the entity state
could also be Added , Deleted , Detached , or Unchanged , you only want to check the
OrderStatus property when it is appropriate based on your business rules. You also want to
access the OriginalValues property only when the original values are actually available;
the property throws the InvalidOperationException when the entity state is Added .
Handling DBNull Values
The OriginalValues property of the ObjectStateEntry class returns an object of
type DbDataRecord , which is shared by the Entity Framework with classic ADO.NET.
Unfortunately, this means that the original null values are returned as objects of type
DBNull and the code has to be prepared to handle them accordingly. In particular,
this means that you cannot simply cast the original OrderStatus value to type
Nullable<Byte> because DBNull values cannot be converted to this type and throw an
InvalidCastException .
One workaround for this problem is to store OrderStatus values in variables of type Object
and to use the static Object.Equals method to compare them. Because it takes the object
type into account, you can safely compare values of type DBNull and Nullable<Byte> for
equality. Here is a new version of the ValidateOrderStatus method, which now correctly
handles the scenario where the original value of the OrderStatus is DBNull :
public static ValidationResult ValidateOrderStatus(
byte? newStatus, ValidationContext context)
{
var stateEntry = (ObjectStateEntry)
context.Items[typeof(ObjectStateEntry)];
if (stateEntry.State == EntityState.Modified)
{
object oldStatus = stateEntry.OriginalValues[“OrderStatus”];
if (object.Equals(oldStatus, (byte)DataModel.OrderStatus.Fulfilled)
&& !object.Equals(newStatus, (byte)DataModel.OrderStatus.Fulfilled))
{
return new ValidationResult(
“Status of fulfilled orders should not be modified”,
new string[] { “OrderStatus” });
}
}
return ValidationResult.Success;
}