Updating Values In A HashTable Loop

In VB .Net

For idx As Integer = 0 To HashTable.Length -1

    Dim ot As ObjectType  = CType(HashTable(idx), ObjectType)

Next

FernandoSoto: That can give some unpredictable results depending on how you are ammending the values an add can change the order of the indexing.

He must be adding/removing keys since he is getting an error on the enumerator (this code wouldn't cause a problem with a key enumerator).



Could you please put up your actual code that is failing?

Thanks,

Greg

@ gregoryyoung;

Yes you are correct about adding to the hash table but the question states that he wants to amend the values he did not say anything about adding any new items.

Fernanso  I get the error that Length is not a member of HashTable. (VB.NET 2008, .NET 2.0)

Sorry it should be Count and not Length

For idx As Integer = 0 To HashTable.Count -1

@fernando my point is that if he is not he can just use a foreach on keys ;-)

Cheers,

Greg

Fernando - can't get the (iIdx) part of "Dim ot As ObjectType  = CType(HashTable(idx), ObjectType)" to return anything sensible
Greg -  appreciate some example VB.NET code showing to do this.

Even though Count=1, HashTable(0) always returns 'Nothing'

@cehrnow:

I asked you to put up your code and I would change your code to fix it.

Greg

For iIdx = 0 To  myHashTable.Count - 1
    obj = myHashTable(iIdx)    'aways nothing
Next

I meant your original code for what you are trying to actually do.


btw: you can't do what you are doing there ... it is looking up the item by the key of iIdx which is incorrect.

If you put up your original non-working code we can just fix it for you.

Cheers,

Greg

' essentially this
For Each Item In myHashTable
     oMyRecord = Item.Value
     sKey = oMyRecord.KeyPart1 & "," & oMyRecord.KeyPart2
     oRecord.MyData="New"
     myHashTable(sKey) = oRecord
Next

OK! here we see the issue! You are changing the key of the record ...

     myHashTable(sKey) = oRecord

This invalidates the enumerator ...

Are you actually changing the data in the key? If not you could just do this ...

For Each Item In myHashTable
     oMyRecord = Item.Value
     oMyRecord.MyData="New"
Next

and it won't break. If you are changing the key I can show you how to deal with that as well...

Cheers,

Greg

I've tried this Greg but oMyRecord appears to be a copy and not a reference and therefore oMyRecord gets updated but not the underlying HashTable value. btw. not trying to change any keys

is oMyRecord a struct? are you in 2.0? or higher?

yes it is
using VS2008 but targeting 2.0

Ah then you can't avoid invalidating the enumerator (you are setting to a new value).

In 2.0 or higher you should not be using a hashtable though ... you should be using a Dictionary<T,V> (its like 10 times faster for structs as it doesn't require boxing/unboxing like the hashtable does).

Here is the code for use with a hashtable.



For Each Key In myHashTable.Keys
     oMyRecord = myHashTable(Key)
     oRecord.MyData="New"
     myHashTable(Key) = oRecord
Next

You can work in a similar manner with a Dictionary ...

Cheers,

Greg

With your last code still get {"Collection was modified; enumeration operation may not execute."}
Thanks

Also, given that it would have to "look-up" each record in each loop do you think there's much performance advantage anyway over an ArrayList ?