How to add control dyanamically in asp.net and how to this control create events

when you have added control dyanamically then how to implements code just like if you are create dyanamic control text box then how to implements code for example text change event or if you have assign any value to that text box so how to get this value.

on server event
for example on dropdown product event
drdl_product_selectedindexchangedevent(object sender,eventergs e)
{
int i=convert.toint31(drdl.selectedvalue);
// consider i=4
for(int i=0;i<4;i++)
textbox tx=new textbox();
//consider in aspx page i have add div_control for adding this text box
div_control.add(tx);

}

when i run this program 4 text box created my problem is if i am added any values on this text box so
how to get this values in btn click event and if i want to create text change events in this text box so how to create event for this text box on server side.

Thanks in advanced !!