C# DataTable to JSON

Utils.cs

public class Utils
{
    public static object DataTableToObject(DataTable dataTable)
    {
        var list = new List<object>();
        foreach (DataRow row in dataTable.Rows)
        {
            var pairList = new List<KeyValuePair<string, object>>();
            foreach (DataColumn column in dataTable.Columns)
            {
                var value = row[column];
                if (column.DataType == typeof(DateTime))
                {
                    value = ((DateTime)row[column]).ToString("yyyy-MM-dd hh:mm:ss");
                }
                var pair = new KeyValuePair<string, object>(column.ColumnName, value);
                pairList.Add(pair);
            }
            list.Add(pairList.ToDictionary(x => x.Key, x => x.Value));
        }
        return list;
    }
}

web.aspx.cs

[WebMethod(EnableSession=true)]
public static object GetFreePhoto(string month, string volume)
{
    var conn = new SqlConnection(ConfigurationManager.ConnectionStrings["SqlConnectionString"].ConnectionString);
    conn.Open();

    var sqlCommand = new SqlCommand
    {
        Connection = conn,
        CommandText = "select * from dbo.iva_free_photo where month=@month and volume=@volume"
    };
    sqlCommand.Parameters.AddWithValue("@month", month.Substring(0, 4) + "-" + month.Substring(4, 2) + "-01");
    sqlCommand.Parameters.AddWithValue("@volume", volume);
    var sda = new SqlDataAdapter(sqlCommand);
    var ds = new DataSet();
    sda.Fill(ds);
    var result = Utils.DataTableToObject(ds.Tables[0]);
    conn.Close();
    return result;
}