Retain values of List between page postbacks

I am working on this code and I realised that the List gets initialized everytime from a different event handler. I do not want the List to be initialized to empty, I need the value to be retained. I have placed the declaration of the list in the class instead of in the method. Please go through my code and correct me.

Hi, I have edited my post, did some reading and realised that it is very much connected to retaining values between page postbacks.

public partial class WebForm4 : System.Web.UI.Page
{
#region List of Weeks
public List A_Week_1 = new List();
public List A_Week_2 = new List();
public List A_Week_3 = new List();
public List A_Week_4 = new List();
public List A_Week_5 = new List();
public List A_Week_6 = new List();
#endregion
protected void Page_Load(object sender, EventArgs e)
{
}
protected void btnUpload_Click(object sender, EventArgs e)
{
if (FileUpload1.HasFile)
{
try
{
string filename = Path.GetFileName(FileUpload1.FileName);
//FileUpload1.SaveAs(Server.MapPath("~/Uploaded Files/") + filename);
string fileExtension = Path.GetExtension(FileUpload1.PostedFile.FileName);
string fileLocation = Server.MapPath("~/Uploaded Files/" + filename);
FileUpload1.SaveAs(fileLocation);
lblMessage.Text = "Upload status: File uploaded to " + fileLocation;
readfile();
}
catch (Exception ex)
{
lblMessage.Text = "Upload status: The file could not be uploaded. The following error occured: " + ex.Message;
}
}
}
protected void readfile()
{
//datasource
string connString = ConfigurationManager.ConnectionStrings["testexcel"].ConnectionString;
//object represents a unique connection to a data source.
OleDbConnection oledbConn = new OleDbConnection(connString);
try
{
// Open connection
oledbConn.Open();
DataTable dtsheet = new DataTable("storesheettable");
//because the oledbConn.GetOlddbSchemaTable returns datatale
dtsheet = oledbConn.GetOleDbSchemaTable(OleDbSchemaGuid.Tables, null);
//initialize and call constructor
if (dtsheet == null)
{
Label1.Text = "No sheet to display";
}
else
{
//String[] excelSheets = new String[dtsheet.Rows.Count];
List<string> import_excel_weeks_list = new List<string>();
//int i = 0;
char[] charsToTrim = { '$' };
for (int i = 0; i < dtsheet.Rows.Count; i++)
{
//excelSheets[i] = dtsheet.Rows.ToString();
import_excel_weeks_list.Add(dtsheet.Rows[i]["TABLE_NAME"].ToString().Trim(charsToTrim));
}
Repeater1.DataSource = import_excel_weeks_list;
Repeater1.DataBind();
}
}
catch (Exception ex)
{
catchexception.Text = ex.ToString();
}
finally
{
// Close connection
oledbConn.Close();
}
}
protected void checkcalendarweek(string weekname)
{
if (weekname == "Week2")
{
int storemonth;
DateTime today = new DateTime();
storemonth = today.Month;
Response.Write(storemonth);
}
}
public void monthsddl_SelectedIndexChanged(object sender, EventArgs e)
{
int total_no_days;
int selected_month = Convert.ToInt32(monthsddl.SelectedValue);
//total number of days in the month for the year
total_no_days = DateTime.DaysInMonth(DateTime.Today.Year, selected_month);
Label1.Text = String.Format("Total number of days in the month of {0} is {1}", monthsddl.SelectedItem,total_no_days);
DateTime firstOfMonth = new DateTime(Convert.ToInt32(DateTime.Now.Year), selected_month, 1);
int startdayofthemonth = (int)DayOfWeek.Monday;
//total weeks in a month
int weeksInMonth = (int)Math.Ceiling((startdayofthemonth + total_no_days) / 7.0);
int weeknum = 0;
//dates are of course from 1 to the total_no_days
for (int i = 1; i <= total_no_days; i++)
{
//create the date based on the selected month with i starting as 1
DateTime date1 = new DateTime(DateTime.Today.Year, selected_month, i);
DateTime dateonly = date1.Date;
//get the local culture timeformat
DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
//gets the calendar for the local timeformat and assign it to a type of calendar, gregorian
System.Globalization.Calendar cal = dfi.Calendar;
int num = cal.GetWeekOfYear(date1, dfi.CalendarWeekRule, (DayOfWeek.Monday));
//week in a month
DateTime test = new DateTime(date1.Year, date1.Month, 1);
int num2 = cal.GetWeekOfYear(test, dfi.CalendarWeekRule, (DayOfWeek.Monday));
weeknum = num - num2 + 1;
//input dates for the week
if (weeknum == 1)
{
A_Week_1.Add(date1);
}
else if(weeknum == 2)
{
//week2.Add(new Week() { datefortheweek = date1 });
A_Week_2.Add(date1);
}
else if(weeknum == 3)
{
//week3.Add(new Week() { datefortheweek = date1 });
A_Week_3.Add(date1);
}
else if (weeknum == 4)
{
//week4.Add(new Week() { datefortheweek = date1 });
A_Week_4.Add(date1);
}
else if (weeknum == 5)
{
//week5.Add(new Week() { datefortheweek = date1 });
A_Week_5.Add(date1);
}
else
{
//week6.Add(new Week() { datefortheweek = date1 });
A_Week_6.Add(date1);
}
Response.Write(weeknum);
Response.Write(" ");
Response.Write(dateonly.ToString("dd/M/yyyy"));
Response.Write(" ");
Response.Write(dateonly.DayOfWeek);
Response.Write("
");
}
//displaying the list
foreach (DateTime text in A_Week_5)
{
Response.Write(text);
Response.Write("
");
}
//Response.Write("Test" + A_Week_5.Last().datefortheweek);
Response.Write("Test" + A_Week_5.Last());
//createtable(weeksInMonth, week1.First().datefortheweek, week1.Last().datefortheweek);
createtable(weeknum, A_Week_1, A_Week_2, A_Week_3, A_Week_4, A_Week_5, A_Week_6);
}
protected void createtable(int pass_in_weeksInMonth, List week_1, List week_2, List week_3, List week_4, List week_5, List week_6)
{
DataTable createweek_dates = new DataTable();
DataColumn column_createweek_dates;
DataRow rows_createweek_dates;
//Column 1
column_createweek_dates = new DataColumn();
column_createweek_dates.DataType = Type.GetType("System.Int32");
column_createweek_dates.ColumnName = "Week Number";
column_createweek_dates.ReadOnly = true;
column_createweek_dates.Unique = true;
// Add the Column to the table
createweek_dates.Columns.Add(column_createweek_dates);
//Column 2
column_createweek_dates = new DataColumn();
column_createweek_dates.DataType = Type.GetType("System.DateTime");
column_createweek_dates.ColumnName = "Start Date";
column_createweek_dates.ReadOnly = true;
//column_createweek_dates.Unique = true;
// Add the Column to the table
createweek_dates.Columns.Add(column_createweek_dates);
//Column 3
column_createweek_dates = new DataColumn();
column_createweek_dates.DataType = Type.GetType("System.DateTime");
column_createweek_dates.ColumnName = "End Date";
column_createweek_dates.ReadOnly = true;
//column_createweek_dates.Unique = true;
// Add the Column to the table
createweek_dates.Columns.Add(column_createweek_dates);
for (int i = 1; i <= pass_in_weeksInMonth; i++)
{
//new row for every creation
rows_createweek_dates = createweek_dates.NewRow();
rows_createweek_dates["Week Number"] = i;
for (int j = i; j <= i; j++)
{
if (j == 1)
{
rows_createweek_dates["Start Date"] = week_1.First();
rows_createweek_dates["End Date"] = week_1.Last();
}
else if (j == 2)
{
rows_createweek_dates["Start Date"] = week_2.First();
rows_createweek_dates["End Date"] = week_2.Last();
}
else if (j == 3)
{
rows_createweek_dates["Start Date"] = week_3.First();
rows_createweek_dates["End Date"] = week_3.Last();
}
else if (j == 4)
{
rows_createweek_dates["Start Date"] = week_4.First();
rows_createweek_dates["End Date"] = week_4.Last();
}
else if (j == 5)
{
rows_createweek_dates["Start Date"] = week_5.First();
rows_createweek_dates["End Date"] = week_5.Last();
}
else
{
rows_createweek_dates["Start Date"] = week_6.First();
rows_createweek_dates["End Date"] = week_6.Last();
}
}
createweek_dates.Rows.Add(rows_createweek_dates);
}
rptItemsInCart.DataSource = createweek_dates;
rptItemsInCart.DataBind();
}
public void DoSend(object sender, EventArgs e)
{
AllLists create1 = new AllLists();
for (int i = 0; i < rptItemsInCart.Items.Count; i++)
{
//Retrieve the state of the CheckBox
CheckBox cb = (CheckBox)rptItemsInCart.Items[i].FindControl("CheckBox1");
if (cb.Checked)
{
//Retrieve the value associated with that CheckBox
HiddenField hiddentext = (HiddenField)rptItemsInCart.Items[i].FindControl("HiddenField1");
//Now we can use that value to do whatever we want
Label2.Text = hiddentext.Value;
create1.user_select_weeks_list.Add(hiddentext.Value);
}
}
selected_weeks_repeater.DataSource = create1.user_select_weeks_list;
selected_weeks_repeater.DataBind();
foreach(DateTime dates1 in A_Week_1)
{
Response.Write(dates1);
}
}
public void Week_button_Click(object sender, EventArgs e)
{
DataTable excel_data_gd = new DataTable();
DataColumn edg_column;
DataRow edg_row;
//get the data from repeater control
AllLists create1 = new AllLists();
for(int i = 0; i < selected_weeks_repeater.Items.Count; i++)
{
Label pass_as_label = (Label)selected_weeks_repeater.Items[i].FindControl("store_user_sheet");
create1.user_select_weeks_list.Add(pass_as_label.Text);
}
//for(int j = 0; j < create1.user_select_weeks_list.Count; j++)
//{
// Response.Write(create1.user_select_weeks_list[j]);
//}
// Column1 - Machine Name
edg_column = new DataColumn();
edg_column.DataType = System.Type.GetType("System.String");
edg_column.ColumnName = "Machine Name";
edg_column.ReadOnly = true;
edg_column.Unique = true;
// Add the Column to the DataColumnCollection.
excel_data_gd.Columns.Add(edg_column);
string[] machinename_array1 = Import_xml_file();
for (int i = 0; i < machinename_array1.Length; i++)
{
edg_row = excel_data_gd.NewRow();
edg_row["Machine Name"] = machinename_array1[i];
//import from excel the value
excel_data_gd.Rows.Add(edg_row);
}
//check if the user has selected any value or not
if(create1.user_select_weeks_list.Contains("Week2"))
{
//check got how many dates within the week
int count_of_dates = A_Week_2.Count;
// The dates column, create a new column for every date within the week
for(int i = 0; i < count_of_dates; i++)
{
edg_column = new DataColumn();
edg_column.DataType = System.Type.GetType("System.DateTime");
edg_column.ColumnName = A_Week_2[i].ToString();
edg_column.ReadOnly = true;
edg_column.Unique = true;
// Add the Column to the DataColumnCollection.
excel_data_gd.Columns.Add(edg_column);
}
}
GridView1.DataSource = excel_data_gd;
GridView1.DataBind();
}
protected string[] Import_xml_file()
{
XmlDocument doc = new XmlDocument();
doc.Load(Server.MapPath("~/App_Data/List_of_machines.xml"));
//Display all the Activities alone in the XML
XmlNodeList elemList = doc.GetElementsByTagName("machine_name");
string[] machinename_array = new string[elemList.Count];
for (int i = 0; i < elemList.Count; i++)
{
machinename_array[i] = elemList[i].InnerXml;
}
return machinename_array;
}
}

What is the best way of retaining the values, ViewState or Session, I read that viewstate if used with large data can cause the page to load slower, how can I go about solving this.