F#: Downcast seq to IEnumerator

Question

Why is it that in F#, I can do this...

let s = seq { for i in 0 .. 4095 do yield i } :?> IEnumerator

... but this throws a System.InvalidCastException?

let s = Seq.init 4095 (fun i -> i) :?> IEnumerator

Answer 1

A sequence expression creates an object that implements IEnumerable<T> and IEnumerator<T>

let s = seq { for i in 0 .. 4095 do yield i }
printfn "%b" (s :? IEnumerable<int>) // true
printfn "%b" (s :? IEnumerator<int>) // true

But Seq.init does not:

let s = Seq.init 4095 (fun i -> i)
printfn "%b" (s :? IEnumerable<int>) // true
printfn "%b" (s :? IEnumerator<int>) // false

You could refactor your code to use IEnumerable<T> instead of IEnumerator since both constructs produce an IEnumerable<T>.

Alternatively, if you really want an IEnumerator, you could simply call GetEnumerator to return an Enumerator from an Enumerable:

let s = (Seq.init 4095 (fun i -> i)).GetEnumerator()
printfn "%b" (s :? IEnumerable<int>) // false
printfn "%b" (s :? IEnumerator<int>) // true