Add values to an existing session without removing values - asp.net

Question

I am looking into adding new values to an existing session. That session will have values in it and I hope that when I add these new ones into it, it would not remove whatever values that is already in that session. E.g. Before adding, the session has 1 value in it. After adding, it should show 2 values in it.
This is what I have tried:
List<string> toAdd = Session["SendData"] as List<string> ??new List<string>();
toAdd.Add(TxtData.Text.Trim());
Session["behzadList"] = behzadList;
I know that there is already some solutions regarding to this problem( the code above is 1 of them) However, when I tried this code, whatever values that were already in the session is being removed and the new values that I have just add would appear instead. So I am not too sure is there any other ways to solve this problem.
Thanks in advance.

Answer 1

List<string> toAdd = Session["SendData"] as List<string> ??new List<string>();
toAdd.Add(TxtData.Text.Trim());
Session["SendData"] = toAdd; // this was wrong it seems. You dont add to the same
session

Answer 2

So, I managed to solve it myself. I used 3 datatables, 1 to store the session values, 1 to store the values that I am going to add in. The last one, is used to merge both datatables values together.
Reason why so many datatables is actually based on your needs and your functions.
For me, it is actually not to confuse the user. I previously only used only 1 datatable. The steps are almost the same as the one I am showing below, but only difference is that you don need to do merging. However, when I add in the new values, it will display not just the values added, but also the values that were already in the session.
This is what I have changed:
//First store session values into the first datatable
DataTable dt = (DataTable)Session["SendData"]
//Create a second datatable to store the values you are going to add in
Datatable DT = new DataTable();
// Do whatever you need to fill this datatable with the values that are going to be added in.
//E.g. add columns, then use datarow to add into datatable or even sql query then fill etc.
//After that create a third datatable to do the merging
DataTable DT3 = new DataTable();
//At this point is where you will merge 2 datatables together.
DT3 = dt.Copy();
DT3.Merge(DT);
//The session will linked be the third datatable instead as it has all the values that you want
Session["SendData"] = DT3;
It kinda looks lengthly but I have to do the explaination as it can be confusing. You could possibly change some things here and there depending on what you want to do with the code. To anyone that may need this for some reference, hope this helps.
To the people that help me by commenting on this post, thanks for giving me suggestions.