type SomeClass(thesize, bytestream : seq<byte>) = class
    let mutable myarray_ = Array.create<byte> thesize 0uy

    do
        let mutable idx = 0
        for v in bytestream do
            myarray_.[idx] <- v
            idx <- idx + 1

    member x.Func(index) = // consumes myarray_.[index] and neighbor values


type OuterClass(thesize, bytestream) = class
    member x.cp : SomeClass = new SomeClass(thesize, bytestream)
    member x.Func(index) =
        x.cp.Func(index)

type SomeClass(thesize, bytestream : seq<byte>) =
    let myarray_ = [| yield! bytestream
                      for i in (Seq.length bytestream)..(thesize-1) -> 0uy |]
    ....

member x.cp : SomeClass = new SomeClass(thesize, bytestream)

type OuterClass(thesize, bytestream) =
    let cp = new SomeClass(thesize, bytestream)
    member x.cp = cp

type SomeClass(size, bytes : seq<byte>) =
    let buf = Array.zeroCreate size
    do
        // Here code assumes that size is always greater than number of items in bytes, is it always correct ?
        Seq.iteri(fun i b -> buf.[i] <- b) bytes
    member this.Buffer = buf

let v = SomeClass(5, (List.init 3 byte))
printfn "%A" v.Buffer // printf [|0uy; 1uy; 2uy; 0uy; 0uy|]