Adding a Javascript function from code-behind in Page_Load()

Question

HI,

 

Why must we add Javascript function from code behind only at the Page_load function

public partial class WebForm5 : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
            //this works
            TextBox1.Attributes.Add("onkeydown", "javascript:alert('keydown')");
            //TextBox1.Attributes.Add("onchange", "javascript:alert('change')");
        }

        protected void TextBox1_TextChanged(object sender, EventArgs e)
        {
             //this does not
            TextBox1.Attributes.Add("OnKeyDown", "alert('Hello')");
            //TextBox1.Attributes.Add("onkeydown", "javascript:alert(this);");
        }
    }

 The first example added to the Page_Load() works whereas the second one does not