阅读器关闭时 FieldCount 的尝试无效

public SqlDataReader  GetCategories()
{
    string connectionString = 
    ConfigurationManager.ConnectionStrings["SQLConnString"].ConnectionString;
    SqlConnection conn = new SqlConnection(connectionString);
    conn.Open();    string cmdString = "SELECT Name FROM Category";
    SqlCommand cmd = new SqlCommand(cmdString, conn);
    SqlDataReader rdr = cmd.ExecuteReader();    //这个怎么办呢？不加，不规范；加呢？阅读器关闭时 FieldCount 的尝试无效。
    conn.Close();    return rdr;
}