C# windows forms listbox selected index changed event fires on form load and button click -

i'm coding program allows user search customers order using customers name user types in name in textbox , search results displayed in listbox control user has select name list box , customer's orders displayed in datagridview problem once customer clicks search button selectedindex changed event fires , causes program crash.

private void btnsearch_click(object sender, eventargs e) {    string query = "select  customerid, companyname customers (companyname ”+                “like'%"+ txtsearch.text + "%')";    clsdatatools.cmdcomand = clsdatatools.con.createcommand();    clsdatatools.cmdcomand.commandtext = query;     clsdatatools.dtadataadapter = new sqldataadapter();    clsdatatools.dtadataadapter.selectcommand = clsdatatools.cmdcomand;    dsordersbycusname = new dataset();    clsdatatools.con.close();    clsdatatools.con.open();    clsdatatools.dtadataadapter.fill(dsordersbycusname);    clsdatatools.con.close();    dsordersbycusname.tables[0].tablename = "orderbcusname";     lstresults.datasource = dsordersbycusname.tables[0];     lstresults.displaymember = "companyname";    lstresults.valuemember = "customerid"; }  private void lstresults_selectedindexchanged(object sender, eventargs d) {     string query = "select  * orders custometid  = '"                     + lstresults.selectedvalue                      + "'";      datagriddataset = new dataset();     clsdatatools.dtadataadapter = new sqldataadapter();     clsdatatools.cmdcomand = clsdatatools.con.createcommand();     clsdatatools.cmdcomand.commandtext = query;     clsdatatools.con.close();     clsdatatools.con.open();     clsdatatools.dtadataadapter.selectcommand = clsdatatools.cmdcomand;     clsdatatools.dtadataadapter.fill(datagriddataset);     clsdatatools.con.close();     datagriddataset.tables[0].tablename = "orders";      dgvcusorders.datasource = datagriddataset;     dgvcusorders.datamember = datagriddataset.tables["orders"].tostring();   } 

i don't understand why selctedindexchanged event fires when click search button there missing maybe?

since set datasource of lstresults call selectedindexchanged, can below

private void btnsearch_click(object sender, eventargs e) {     //remove handler      this.lstresults.selectedindexchanged -= lstresults_selectedindexchanged;     //      // code     //     this.lstresults.selectedindexchanged += lstresults_selectedindexchanged; // add handler  }