Retrive Data From Dynemic Control

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ASP.NET

Over a button Click a panel get display with 5 or more textBox (TextBox is created by for loop according to no of row affected by query.) and one button.
Just
I want's that when i click over panel button then the value of textBox get Stored inside a string seperated by ','.

I did that but The textboxes are lossing there value at roundTrip.
I am not able to handle this so plz help me...
Byyy.

protected void ButtoncreateTextBox_Click(object sender, EventArgs e)
   {
       for (int i = 0; i < 5; i++)
       {
           TextBox t = new TextBox();
           t.ID = "TextBox" + i.ToString();
           Panel1.Controls.Add(t);
       }

   }

I have to retrive data form these text box(added in panel) over other button click.