Optimal use of foreach

When using foreach if the collection is a property will it be called on each
pass or only once?

Can I do:

foreach (string key in myDictionary.Keys)
{
}

or will that call Keys each time, therefore requiring that I do:

Dictionary<string, string>.KeysCollections keys;

keys = myDictionary.Keys;

foreach (string key in keys)
{
}

Thanks,
-Mont

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