TextChanged event is not firing on dynamically created textbox in asp.net c#

c1 = new TableCell();
c1.VerticalAlign = VerticalAlign.Middle;
cell.Width = 80;
TextBox txtFirst = new TextBox();
txtFirst.ID = "TextBoxRow_" + num_row  + "Col_" + c;
txtFirst.EnableViewState = true;
txtFirst.AutoPostBack = true;
txtFirst.TextChanged += new EventHandler(OnTextChanged);

protected void OnTextChanged(object sender, EventArgs e)
{
  TextBox txtFirst = (TextBox)sender;
  string ID = txtFirst.ID;
  ClientScript.RegisterClientScriptBlock(this.GetType(), "Alert",
    "<script type = 'text/javascript'>alert('" + ID +
    " fired OnTextChanged event');</script>");

  
}