Iterating multiple times over a taskSeq { ... } raises InvalidOperationException: An attempt was made to transition a task to a final state when it had already completed.

[abelbraaksma]

As in the title. Any of these errors only happen when multiple iterations are attempted and the source is a taskSeq CE, not when the source is a user-defined or library defined IAsyncEnumerator<_>, used with the TaskSeq library functions.

I already started investigating this issue and it has to do with properly resetting state when "reaching the end" and when "getting an enumerator over the same resource". See #36 continued: #42.

Operation not valid error, MoveNextAsync()

// this throws: InvalidOperationException: Operation is not valid due to the current state of the object.
task {
    let tskSeq = taskSeq { yield 1; yield 2 }
    let enum = tskSeq.GetAsyncEnumerator()
    let! isNext = enum.MoveNextAsync()  // true
    let! isNext = enum.MoveNextAsync()  // true
    let! isNext = enum.MoveNextAsync()  // false
    let! isNext = enum.MoveNextAsync()  // error here
    ()
}

Operation not valid error, multiple GetAsyncEnumerator() with MoveNextAsync()

// throws: InvalidOperationException: Operation is not valid due to the current state of the object.
task {
    let tskSeq = getEmptyVariant variant
    use enumerator = tskSeq.GetAsyncEnumerator()
    let! isNext = enumerator.MoveNextAsync()
    use enumerator = tskSeq.GetAsyncEnumerator()
    let! isNext = enumerator.MoveNextAsync()  // throws here
    ()
}

Transition State error

// this throws: 
// InvalidOperationException: 
// An attempt was made to transition a task to a final state when it had already completed.
task {
    let tskSeq = taskSeq { yield 1; yield 2 }
    let ts1 = tskSeq |> TaskSeq.map (fun i -> i + 1)
    let result1 = TaskSeq.toArray ts1
    let ts2 = ts1 |> TaskSeq.map (fun i -> i + 1)
    let result2 = TaskSeq.toArray ts2 // error here
    ()
}