The problem with your using statement as it inside a loop.
for (int i = 0; i < 11; i++)
{
try
{
string text = System.IO.File.ReadAllText("diff"+i+".sql");
using(SqlConnection conn = new SqlConnection(cString.connString()))
{
conn.Open();
SqlCommand cmd = new SqlCommand(text, conn);
cmd.ExecuteNonQuery();
}
}
catch { MessageBox.Show("No file diff" + i); }
}