Question: i am using this script in .cs page..

```
public void messagebox(string msg)
{
  Label lbl = new Label();
  lbl.Text = "<script language='javascript'>" + Environment.NewLine + "window.alert('" + msg + "')</script>";
  Page.Controls.Add(lbl);
}

```

error as

The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

Answer: 
To register a script, use ScriptManager

```
ScriptManager.RegisterStartupScript(this, this.GetType(), "registeredAlert", lbl, false);

```
This has nothing to do with Javascript. The error message is pretty clear; you're not allowed to add any more controls to Page.Controls at that point in your code.