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Problem
You need to reload those dynamically created textboxes on post back. Otherwise, they will become null, and you won't be able to find it.
In order to do that, you need to save those dynamically TextBoxes Ids in persistent location such as View State or Session State.
Screen Shot
ASPX
Number of TextBoxes: <asp:TextBox runat="server" ID="CounterTextBox"
OnTextChanged="CounterTextBox_TextChanged" AutoPostBack="True" /><br/>
<asp:PlaceHolder runat="server" ID="TextBoxPlaceHolder" /><br/>
<asp:Button runat="server" ID="ConfirmButton" Text="Confirm"
OnClick="ConfirmButton_Click" /><br/>
Result: <asp:Literal runat="server" ID="ResultLiteral"/>
Code Behind
private List<string> TextBoxIdCollection
{
get
{
var collection = ViewState["TextBoxIdCollection"] as List<string>;
return collection ?? new List<string>();
}
set { ViewState["TextBoxIdCollection"] = value; }
}
protected void Page_Load(object sender, EventArgs e)
{
foreach (string textboxId in TextBoxIdCollection)
{
var textbox = new TextBox {ID = textboxId};
TextBoxPlaceHolder.Controls.Add(textbox);
}
}
protected void CounterTextBox_TextChanged(object sender, EventArgs e)
{
var collection = new List<string>();
int total;
if (Int32.TryParse(CounterTextBox.Text, out total))
{
for (int i = 1; i <= total; i++)
{
var textbox = new TextBox { ID = "QuestionTextBox" + i };
// Collect this textbox id
collection.Add(textbox.ID);
TextBoxPlaceHolder.Controls.Add(textbox);
}
TextBoxIdCollection= collection;
}
}
protected void ConfirmButton_Click(object sender, EventArgs e)
{
foreach (Control ctr in TextBoxPlaceHolder.Controls)
{
if (ctr is TextBox)
{
string value = ((TextBox)ctr).Text;
ResultLiteral.Text += value;
}
}
}