I have a single table in a database called Users

Users
------
ID (PK, Identity)
Username (Unique Index)

I have setup a unique index on the Username table to prevent duplicates. I am then enumerating through a collection and creating a new user in the database for each item.

What I want to do is just insert a new user and ignore the exception if the unique key constraint is violated (as it's clearly a duplicate record in that case). This is to avoid having to craft where not exists kind of queries.

First off, is this going to be any more efficient or should my insert code be checking for duplicates instead? I'm drawn more to the database having that logic as this prevents any other type of client from inserting duplicate data.

My other issue is related to LINQ To SQL. I have the following code:

public class TestRepo
{
    DatabaseDataContext database = new DatabaseDataContext();

    public void Add(string username)
    {
        database.Users.InsertOnSubmit(new User() { Username = username });
    }

    public void Save()
    {
        database.SubmitChanges();
    }
}

And then I iterate over a collection and insert new users, ignoring any exceptions:

TestRepo repo = new TestRepo();

foreach (var name in new string[] { "Tim", "Bob", "John" })
{
    try
    {
        repo.Add(name);
        repo.Save();
    }
    catch {  }
}

The first time this is run, great I have three users in the table. If I remove the second one and run this code again, nothing is inserted. I expected the first insert to fail with the exception, the second to succeed (as I just removed that item from the DB) and the third to then fail.

What seems to be happening is that once the SqlException is thrown (even though the loop continues to iterate) all of the next inserts fail - even when there isn't a row in the table that would cause a unique violation.

Can anyone explain this?

P.S. The only workaround I could find was to instantiate the repo each time before the insert, then it worked exactly as excepted - indicating that it's something to do with the LINQ To SQL DataContext.

Thanks.