How get Event "item Selected" with AutoComplete in C#?

There's no such thing as chosen item Event for a textBox, which I believe you're using for the AutoComplete. What you could do is add a key down event to your textBox. There you could verify if the enter key was pressed (clicking on a suggested link is the same as pressing enter). Something like that:

private void textBox1_KeyDown(object sender, KeyEventArgs e) {
    if (e.KeyData == Keys.Enter) {
        String selItem = this.textBox1.Text;
    }
}

Rather than focusing on detecting if an item from the autocomplete list was selected, instead you should check if the current value of the textbox is in the set of autocomplete entries.

if (txtSerial.AutoCompleteCustomSource.Contains(t.Text))
{
    // Logic to handle an exact match being selected
    ...
}
else
{
    // Update the autocomplete entries based on what was typed in
}

If the user typed in an exact string which happens to be be within the list of autocomplete values -- OR -- they select that value from the autocomplete list -- should this produce any different behavior? I think that in most cases it should not.