vb.net - system.nullReferenceException throwed, while starting a loop in a loop using arrays and dictionaries -

i have system.nullreferenceexcpetion. here code. exception thrown @ marked line. if needed can give inizializing of used arrays , dictionaries.

    public sub mappen()     dim feld(form1.arr1.length) string        integer = 0 form1.arr1.length - 1           feld(i) = form1.datagridview1.rows(i).cells(1).value.tostring           ***for ix integer = 0 form1.spaltennamen.length - 1***              select case feld(i)                 case form1.spaltennamen(ix)                     form1.comboauswahld(form1.spaltennamen(ix)) =             end select          next     next end sub 

the method in module1. spaltennamen initilized in form1

here code

 public class form1   public spaltennamen() string       private sub form1_load(sender object, e eventargs) handles mybase.load       dim table new datatable      table = skmtabelle.slxadriumdevdataset.mappen      dim integer = 0     dim spaltennamen(table.columns.count) string      each column datacolumn in table.columns          spaltennamen(i) = column.columnname         = + 1      next 

edit found 1 bug @ form1 class. spaltennamen declared in formloader , in head of class.

nevertheless big bug cant in mappen() method because if take procedur of form1 out, fill spaltennamen , instead fill spaltennamen in code before runtime, works fine...

replace form1 instance of form1 form1 type.

example

dim f new form1()  feld(i) = f.datagridview1.rows(i).cells(1).value.tostring 

debug

public class form1      friend spaltennamen() string '= nothing aka no instance created.      private sub form1_load(sender object, e eventargs) handles mybase.load          '- here got contradiction.          '    1) first create new instance of `datatable`.         '    2) set should have been return value (datatable) of `mappen`,          '       it's `sub` , not `function`.         dim table new datatable         table = skmtabelle.slxadriumdevdataset.mappen         '                                        |         '               --------------------------         '               |         '              \|/         'public sub mappen()              '- form1 type, not instance.             '- work, 1 of following suggestions:             '    1) move code inside `mappen` , place inside method (as shown here) , replace `form1` `me`.             '    2) change method accept instance of form1 `public sub mappen( f form1)`, change `form1` `f` ,             '       call `mappen(me)`.             '- `arr1` instantiated? `forminstance.arr1 = new object(?-1){}` (replace `object` correct data type)             '- arrays in vb.net use zero-base indexing. here create `feld` array can hold +1 more item             '  `arr1`.             dim feld(form1.arr1.length) string             integer = 0 form1.arr1.length - 1                 '- form1 type, not instance.                 '- sure `rows(i)` exists?                 '- sure want second column? cells(1) <- zero-base indexing makes column 2.                 '  first column have index `0`.                 feld(i) = form1.datagridview1.rows(i).cells(1).value.tostring                 ix integer = 0 form1.spaltennamen.length - 1                     select case feld(i)                         '- form1 type, not instance.                         '- spaltennamen instantiated? `forminstance.spaltennamen = new string(?-1){}`                          case form1.spaltennamen(ix)                             '- form1 type, not instance.                             '- `comboauswahld` instantiated?  `forminstance.comboauswahld = new object(?-1){}` (replace `object` correct data type)                             form1.comboauswahld(form1.spaltennamen(ix)) =                     end select                 next             next         'end sub          dim integer = 0         '- in vb.net, if specify length of array, instantiated. (finally)         '- sure should holde +1 more item number of columns?         dim spaltennamen(table.columns.count) string         each column datacolumn in table.columns             spaltennamen(i) = column.columnname             = + 1         next     end sub  end class