Rows removed from a datatable are also removed fro

Question

I am noticing some strange behavior when working with datatables stored in session variables. Removing a row from a datatable that is created from a session variable appears to also modify the session variable as demonstrated below.

<asp:GridView ID="gvResults" runat="server" DataKeyNames="id">
<Columns>
    <asp:TemplateField ShowHeader="False">
        <ItemTemplate>
            <asp:Button ID="btnSelectUserToRemove" runat="server" CausesValidation="false" CommandName="Remove" Text="Select" CommandArgument='<%# Eval("id") %>' OnCommand="Command" />
        </ItemTemplate>
    </asp:TemplateField>
</Columns>
</asp:GridView>


using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data;

public partial class Test : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            DataTable dtResults = new DataTable();
            dtResults.Columns.Add("id");

            dtResults.Rows.Add();
            dtResults.Rows[0]["id"] = "1";
            dtResults.Rows.Add();
            dtResults.Rows[1]["id"] = "2";

            Session["id"] = dtResults;
            gvResults.DataSource = dtResults;
            gvResults.DataBind();
        }
    }

    protected void Command(object sender, CommandEventArgs e)
    {
        if (e.CommandName == "Remove")
        {
            DataTable dtTemp = (DataTable)Session["id"];
            for (Int32 i = 0; i < dtTemp.Rows.Count; i++)
            {
                if (e.CommandArgument.ToString() == dtTemp.Rows[i]["id"].ToString())
                {
                    dtTemp.Rows[i].Delete(); //Should just remove the row from the datatable.
                }
            }
            dtTemp.AcceptChanges();
        }
        DisplayData();
    }

    protected void DisplayData()
    {
        gvResults.DataSource = (DataTable)Session["id"];
        gvResults.DataBind(); //Should display both rows because I did not save the modified datatable back to this session variable.
    }
}

I am not saving the modified dtTemp datatable back to the Session["id"] variable. It is my understanding that there should be 1 row in dtTemp, because one row was deleted, and 2 rows in Session["id"] when calling DisplayData. However, only one row is displayed in the gridview after DisplayData runs.

I am hoping there is someone out there that can shed some light on why this is happening.

Answer 1

It's because dtTemp and the session variable are object references that both point to the same object.

Answer 2

This line:

DataTable dtTemp = (DataTable)Session["id"];

Does not do what you think it does. Since the DataTable is a reference type, you're just grabbing a pointer to that object's location in memory (which happens to be in the SessionState variable), not creating a fresh copy of it.

If you want to operate on the DataTable without modifying the one in Session, you'll need to look into a Clone pattern, like How do you do a deep copy of an object in .NET (C# specifically)?