Custom Server Controls :: Persist Data In A Dynamically Populated ListBox In A User Control

I am trying to persist data in a dynamically populated ListBox in a user control. Here are the details

I have a user control which contains a a listBox and button. On Page_Load of user control I am populating the ListBox control with some values. On Click of button, I am trying to fetch the values which I have selected in ListBox. Though this seems very simple and straight forward I am unable to fetch values.

My Page_Load Function

void Page_Load(object sender, EventArgs e)
{
If(!IspostBack)
{
listBox.items.Add("Item1");
listBox.items.Add("item2");
listBox.items.Add("item3");
}
}

My Click Function

Protected Void Button1_Click(object sender, EventArgs e)
{
string strValue = listBox.SelectedValue
}

I have placed brake point at Page_Load and on click function, I am seeing that the moment the breakpoint hits Page_Load function (before If (!Ispostback)) , listBox.SelectedIndex is becoming -1.

One more thing here is, I am placing this user control in an SharePoint application page to be specific checkin.aspx page . ( I know this should not be done, but that is the requirement for me)