When does “SqlConnection does not support parallel transactions” happen?

You’ll get this if the connection already has an uncommitted transaction and you call BeginTransaction again.

In this example:

class Program
{
    static void Main(string[] args)
    {
        using (SqlConnection conn = new SqlConnection("Server=.;Database=TestDb;Trusted_Connection=True;"))
        {
            conn.Open();

            using (var tran = conn.BeginTransaction())
            {
                using (var cmd = new SqlCommand("INSERT INTO TESTTABLE (test) values ('" + DateTime.Now.ToString() + "')", conn))
                {
                    cmd.Transaction = tran;
                    cmd.ExecuteNonQuery();
                }

                using (var tran2 = conn.BeginTransaction())    // <-- EXCEPTION HERE
                {
                    using (var cmd = new SqlCommand("INSERT INTO TESTTABLE (test) values ('INSIDE" + DateTime.Now.ToString() + "')", conn))
                    {
                        cmd.Transaction = tran2;
                        cmd.ExecuteNonQuery();
                    }

                    tran2.Commit();
                }

                tran.Commit();
            }
        }
    }
}

… I get exactly the same exception at the second BeginTransaction.

Make sure the first transaction is committed or rolled back before the next one.

If you want nested transactions, you might find TransactionScope is the way forward.