SqlBulkCopy Tutorial SqlBulkCopy - WriteToServer requires an open and available Connection. The connection's current state is closed.

using(var connection = new SqlConnection(FiddleHelper.GetConnectionStringSqlServer()))
{
    // Oops! The connection has never been opened
    //connection.Open();

    using (var sqlBulk = new SqlBulkCopy(connection))
    {		
     	sqlBulk.DestinationTableName = "Customers";
     	sqlBulk.WriteToServer(dt);
    }
}

using(var connection = new SqlConnection(FiddleHelper.GetConnectionStringSqlServer()))
{
    // Make sure that connection is open
    connection.Open();

    using (var sqlBulk = new SqlBulkCopy(connection))
    {		
     	sqlBulk.DestinationTableName = "Customers";
     	sqlBulk.WriteToServer(dt);
    }
}