How i can force the entity framework to raise a unique exception if the Unique key constraint has been violated on thr sql server database - asp.net-mvc-3

Question

I have a table named countries and i define the country_name field to be unique by creating a “Index/Key” of type “Unique Key” on sql servwer 2008 r2.
But currently if the user insert a country_name value that already exists on my asp.net mvc3 application, then an exception of type “System.Data.Entity.Infrastructure.DbUpdateException” will be raised , which is very general.
So is there a way to define a specific exception in case the unique constraint has been violated ??? rather than just raising the general “System.Data.Entity.Infrastructure.DbUpdateException” exception?
BR

Answer 1

Most likely, thought I can't test it at the moment, the inner exception of DbUpdateException is probably an exception about a duplicate or foreign key constraint. More importantly, you have an opportunity to not throw any exceptions by checking to see if a country already exists. Two ways I can think of are to; check and see if the country already exists by doing a simple select, and if it doesn't, doing an insert/add or write a stored procedure that and do a select/insert or merge and return any value(s) you want back.
Update
(this is example code to demonstrate the logic flow of events and not good programming practice, specially by catching all excepts)
Exception Logic
public AddCountry(string countryTitle)
{
using (var db = new DbContext(_connectionString)
{
try
{
// Linq to (SQL/EF)-ish code
Country country = new Country();
country.ID = Guid.NewGuid();
country.Title = countryTitle;
db.Countrys.Add(country);
db.SubmitChanges(); // <--- at this point a country could already exist
}
catch (DbUpdateException ex)
{
// <--- at this point a country could be delete by another user
throw Exception("Country with that name already exists");
}
}
}
Non-Exception Logic
public AddCountry(string countryTitle)
{
using (var db = new DbContext(_connectionString)
{
using (TransactionScope transaction = new TransactionScope())
{
try
{
Country country = db.Countries
.FirstOrDefault(x => x.Title = countryTitle);
if (country == null)
{
country = new Country();
country.ID = Guid.NewGuid();
country.Title = countryTitle;
db.Countrys.Add(country);
db.SubmitChanges(); // <--- at this point a country
// shouldn't exist due to the transaction
// although someone with more expertise
// on transactions with entity framework
// would show how to use transactions properly
}
}
catch (<someTimeOfTransactionException> ex)
{
// <--- at this point a country with the same name
// should not exist due to the transaction
// this should really only be a deadlock exception
// or an exception outside the scope of the question
// (like connection to sql lost, etc)
throw Exception("Deadlock exception, cannot create country.");
}
}
}
}
Most likely the TransactionScope(Transaction transactionToUse) Constructor would be needed and configured properly. Probably with an Transactions.IsolationLevel set to Serializable
I would also recommend reading Entity Framework transaction.