vb.net - Validation to get null values -

i trying make validation if there no value on label application run normally. explain better show the code using , explain it

private sub showtotalresult()     sqlcon = new sqlconnection     sqlcon.connectionstring = "...."     try         sqlcon.open()         dim query string         query = "select                  cast(sum(cast(filesize float)) / 1024 / 1024 decimal(10,2))                 infofile"         sqlcmd = new sqlcommand(query, sqlcon)         sqldr = sqlcmd.executereader         if lblresultadototal.text <> ""             if sqldr.read()                 lblresultadototal.text = sqldr.getdecimal(0)                 exit sub             end if         end if         sqlcon.close()     catch ex sqlexception         msgbox(ex.message)             sqlcon.dispose()     end try end sub 

what piece of code giving me total in mb of got on database. make tests deleted info database , when i've tried run application gives me error on line lblresultadototal.text = sqldr.getdecimal(0) because there no value on label. turn around do?

i've tried use if lblresultadototal.text <> "" lblresultadototal.text = sqldr.getvalue(0) end if

this query using values database

select  cast(sum(cast(filesize float)) / 1024 / 1024 decimal(10,2)) infofile 

this error data null. method or property cannot called on null values. not work. have solution?

you cannot call getdecimal if field null because internally getdecimal method tries cast null value decimal. can read warning in remarks section of getdecimal page on msdn

it easy fix using sqldatareader.isdbnull

' call sqldatareader.read required position ' ' reader on first record returned query' if sqldr.read()      if sqldr.isdbnull(0)          lblresultadototal.text = "0,00"     else          lblresultadototal.text = sqldr.getdecimal(0)      end if end if 

however, bit perplexed casts inside query. raw value , try convert values obtained in vb.net code more easy it. (need test performances)