vb.net - String.Substring(start, end) sometimes throwing an exception -

my program uses backgroundworker call performaction() method when different method, method1 returns true. using strategy pattern set correct performaction() should performed every time event raised.

for strategy pattern created abstract class, , class inside abstract class inherits it.

public  mustinherit class abstract     public mustoverride sub performaction(byval str string)     public class extends            inherits abstract             public overrides sub performaction(byval str string)                 str = str.substring(str.indexof(":"), str.indexof(">"))                 msgbox(str)             end sub end class 

i create class contains field of abstract, , used call performaction.

the performaction method gets called backgroundworker.reportprogress event, called when backgroundworker.dowork detects method1 returning true. , code above, causes system.reflection.targetinvocationexception addition information exception has been thrown target of invocation.

the debugger tells me:

       cannot obtain value of local or argument '<this>' not available @ instruction pointer, possibly because has been optimized away.  system.delegate      args    cannot obtain value of local or argument 'args' not available @ instruction pointer, possibly because has been optimized away.    object[] 

strangely enough, when perform (what seems me) identical operation 2 substrings:

s = s.substring(s.indexof(":")) s = s.substring(0, s.indexof(">")) 

it functions perfectly.

what difference between these 2 methods? inheritance set incorrectly , causing these errors? what's going on here?

let me know if need add more code explain situation. thanks.

to effect of

s = s.substring(s.indexof(":")) s = s.substring(0, s.indexof(">")) 

in single statement, need calculate length of desired substring

s = s.substring(s.indexof(":"), s.indexof(">") - s.indexof(":")) 

note if possible string not contain ":" followed later ">", need first verify indexof(":") >= 0 , s.indexof(">") returns value greater indexof(":").