How do I iterate over a hashtable in F#? - f#

Question

let dic = Environment.GetEnvironmentVariables()
dic
|> Seq.filter( fun k -> k.Contains("COMNTOOLS"))
fails to compile.
I've tried using Array.filter, Seq.filter, List.filter
I've tried getting just the dic.Keys to iterate over but F# doesn't seem to want me to coerce a KeyCollection into an IEnumerable.
I've tried upcasting the hashtable into an IEnumerable<KeyValuePair<string,string>>
How do I walk the hashtable that is returned from Environment.GetEnvironmentVariables() ?

Answer 1

Since Environment.GetEnvironmentVariables() returns a non-generic IDictionary and it stores key/value pairs in DictionaryEntry, you have to use Seq.cast first:
let dic = Environment.GetEnvironmentVariables()
dic
|> Seq.cast<DictionaryEntry>
|> Seq.filter(fun entry -> entry.Key.ToString().Contains("COMNTOOLS"))
See the relevant docs at https://msdn.microsoft.com/en-us/library/system.collections.idictionary(v=vs.110).aspx. Notice that entry.Key is of type obj, so one has to convert to string before checking string containment.
Instead of using high-order functions, sequence expression might be handy:
let dic = Environment.GetEnvironmentVariables()
seq {
for entry in Seq.cast<DictionaryEntry> dic ->
(string entry.Key), (string entry.Value)
}
|> Seq.filter(fun (k, _) -> k.Contains("COMNTOOLS"))

Answer 2

F# Seq can only operate with System.Collections.Generic.IEnumerable<_>. System.IDictionary returned by Environment.GetEnvironmentVariables is not generic but it implements non-generic System.Collections.IEnumerable and not System.Collections.Generic.IEnumerable<_>. System.Collections.IEnumerable does not contain type information and allows enumeration of the collection of boxed types i.e. instances of System.Object.
Anyway System.IDictionary can be enumerated as collection of System.Collections.DictionaryEntry objects so you can simply call Seq.cast on it. It will give you access to Key and Value properties, yet still boxed as objects, so you should unbox them too.
let dic = System.Environment.GetEnvironmentVariables()
dic
|> Seq.cast<System.Collections.DictionaryEntry>
|> Seq.filter( fun k -> (k.Key :?> string).Contains("COMNTOOLS"))
Alternatively you can use the following function
let asStringPairSeq (d : System.Collections.IDictionary) : seq<string * string> =
Seq.cast<System.Collections.DictionaryEntry> d
|> Seq.map (fun kv -> kv.Key :?> string, kv.Value :?> string)
System.Environment.GetEnvironmentVariables()
|> asStringPairSeq
|> Seq.filter (fun (k,v) -> k.Contains("COMNTOOLS"))