c# - system.formatexception input string was not in a correct format when selecting a value in dropdownlist -

everytime pick value meatyqty dropdownlist in other dropdownlist, error "input string not in correct format. here code:

protected void page_load(object sender, eventargs e) {     int x;     (x = 0; x <= 100; x++)     {         listitem item = new listitem(x.tostring(),x.tostring());         tidalqty.items.add(item);         meatyqty.items.add(item);         darknessqty.items.add(item);         macaroniqty.items.add(item);         cheesyqty.items.add(item);         baconqty.items.add(item);         loveqty.items.add(item);     }         }  protected void tidalqty_selectedindexchanged(object sender, eventargs e) {     tidalprice.text = ((double.parse(tidalqty.selectedvalue)) * 250).tostring();     computetotal(); }  protected void meatyqty_selectedindexchanged(object sender, eventargs e) {     meatyprice.text = ((double.parse(meatyqty.selectedvalue)) * 500).tostring("f2");     computetotal(); }   void computetotal() {     double sub1 = double.parse(tidalprice.text);     double sub2 = double.parse(meatyprice.text);      subprice.text = (sub1+sub3).tostring("f2");               } 

you're running trouble adding / rebuilding list on each post back, it's having trouble selectedvalue.

try:

protected void page_load(object sender, eventargs e) {     if (!page.ispostback)     {       int x;       (x = 0; x <= 100; x++)       {         listitem item = new listitem(x.tostring(),x.tostring());         tidalqty.items.add(item);         meatyqty.items.add(item);         darknessqty.items.add(item);         macaroniqty.items.add(item);         cheesyqty.items.add(item);         baconqty.items.add(item);         loveqty.items.add(item);       }            }  } 

i've added call page.ispostback.