vb.net - NullReferenceException thrown even though checking for Nothing -

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i have rather straightforward code compare function

    public overridable function comparer(thisvalue object, othervalue object) integer     try         if thisvalue nothing             if othervalue nothing                 return 0             else                 return -1             end if         else             if othervalue nothing                 return 1             else                 return thisvalue.tostring.compareto(othervalue.tostring)             end if         end if     catch ex exception         return 0     end try end function

the reason try-catch block is: nullreferenceexception @ actual comparision line if thisvalue nothing. debugger shows me thisvalue "nothing" lands in else branch anyway.

can tell me why?

update: have tried amend situation inserting yet check nothing. in scenario boils down few hundred exceptions , execution speed bearable. don't want imagine trying sort empty column though.

http://i.stack.imgur.com/8dnxd.png

how possible? there "level" of nothingness don't know about. need check type of thisvalue , othervalue?

the function never overridden. have tried removing "overridable" no effect.

perhaps isn't thisvalue nothing, fact .tostring() returning nothing? try code test out:

public overridable function comparer(thisvalue object, othervalue object) integer     try         if thisvalue nothing             if othervalue nothing                 return 0             else                 return -1             end if         else             if othervalue nothing                 return 1             else                  dim thisvaluestr string = thisvalue.tostring()                 dim othervaluestr string = othervalue.tostring()                  'here, check 2 strings null!!!                  return thisvaluestr .compareto(othervaluestr )              end if         end if     catch ex exception         return 0     end try end function

if case, double-check implementation of tostring() in object being passed (assuming custom type).


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