Preempting NullReferenceExceptions

/// strips values of reference types that are null
let denull<'T when 'T : not struct> (x : 'T) =
    // some F# core types encode values with null, ignore those
    let usesNullAsTrueValue() =
        if typeof<'T> = typeof<unit> then true
        else
            let attrs = typeof<'T>.GetCustomAttributes(typeof<CompilationRepresentationAttribute>, false)
            if attrs.Length = 0 then false
            else
                let flags = (attrs.[0] :?> CompilationRepresentationAttribute).Flags
                flags.HasFlag CompilationRepresentationFlags.UseNullAsTrueValue

    if obj.ReferenceEquals(x, null) && not <| usesNullAsTrueValue() then None
    else Some x

// define the option monad
type OptionBuilder() =
    member __.Return x = Some x
    member __.Bind(x,f) = Option.bind f x
    member __.ReturnFrom (x : 'T option) = x

let option = new OptionBuilder()

// example
open Microsoft.Win32

option {
    let bkey = RegistryKey.OpenBaseKey(RegistryHive.CurrentUser, RegistryView.Default)
    let! k1 = denull <| bkey.OpenSubKey(@"Software\Microsoft\VisualStudio\11.0")
    let! k2 = denull <| k1.OpenSubKey(@"DialogPage\Microsoft.VisualStudio.FSharp.Interactive.FsiPropertyPage")
    let! switch = denull (k2.GetValue "FsiPreferAnyCPUVersion" :?> string)

    if switch = "False" then
        return! denull (k2.GetValue "FsiCommandLineArgs" :?> string)
    else
        return! None
}