What the page load event is for

Every time a page runs, the server raises a load event. It is the natural home for work that must happen whenever the page appears.

Typical uses

• Filling a dropdown list with choices.
• Showing a default value on first visit.
• Reading a value from a database to display.

The trap to avoid

Load runs again on every postback, not only the first visit. If you add items to a list here without checking, the list can end up with duplicates after the visitor clicks a button. Check whether this is the first load before you fill it.

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack)
    {
        sizeList.Items.Add("Small");
        sizeList.Items.Add("Large");
    }
}

A related event fires when a specific control changes, covered in reacting to a change in a text box.