how to used store multiple data in textbox move single data

how to used store multiple data in textbox in saparation in comma ,move single selected word in listbox using c# code

------------------------- Source Code -----------------------------------------

-------------------------- Page Load Code----------------------

DataTable dt=null;

if(ViewState["data"]==null)

{

DataTable dt = new DataTable();

dt.Columns.Add("ColumnID");

ViewState["data"] = dt;

}

----------------------- Button Code-------------------------------

protected void Button1_Click1(object sender, EventArgs e)

{

string str = Textarea1.Value.Trim();

dt = (DataTable)ViewState["data"];

dt.Rows.Add(str);

ViewState["data"] = dt;

List2.DataSource = dt;

List2.DataValueField = "ColumnID";

List2.DataBind();

}

please help me....