Don't, basically. It's explicitly not supported. From the docs for Dictionary<,>.GetEnumerator()
:
An enumerator remains valid as long as
the collection remains unchanged. If
changes are made to the collection,
such as adding, modifying, or deleting
elements, the enumerator is
irrecoverably invalidated and its
behavior is undefined.
Typically the best way is to remember the modifications you want to make, and then perform them afterwards. Or you could take a copy of the dictionary to start with and then iterate through that while you modify the original. If you could give us more information about what you're trying to do, that would help.
Having said this, the new ConcurrentDictionary
class in .NET 4.0 does permit this - but the results of the iteration aren't guaranteed - you may see the changes while you're iterating, or you may not.
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