f＃将obj转换为FSharpValue.MakeTuple中的元组类型

Question

I have an obj which is actually a tuple DateTime*int*decimal*decimal , it is an obj , because I create the tuple by using,

let tupleType = typeof<(DateTime*int*decimal*decimal)>
let ret = FSharpValue.MakeTuple(x, tupleType)

Now I would like to cast ret backto DateTime*int*decimal*decimal , I did,

 ret :?> DateTime*int*decimal*decimal

My question is can I use tupleType to do the casting instead of specifying the type explicitly as in the above code?

.

Answer 1

The short answer is no .

Casting and type checking happens already at compile-time. It's called static typing because the types are independent of the run-time (dynamic) values. The types are already available when the code isn't running (ie is static ).

At compile-time, ret has the static type obj , while at run-time it has the type DateTime*int*decimal*decimal .

The compiler, however, doesn't know this unless you tell it.

Likewise, tupleType has the type Type at both compile-time and run-time. It's value , however, is DateTime*int*decimal*decimal . That information is only available at run-time.

If you want ret to have the type DateTime*int*decimal*decimal , you'll need to tell the compiler that this is the type. At compile-time, tupleType doesn't provide that information.

If you can design your system so that you don't need to use Reflection, you can probably stay type-safe, so if that's at all possible, it'd probably be a better option.

Answer 2

You cannot do it the way you expected, as Mark explained in his answer . However, what you can do is using type abbreviation :

open System
open Microsoft.FSharp.Reflection

type MyTuple = DateTime*int*decimal*decimal

let ret = FSharpValue.MakeTuple([|DateTime.Now;1;1.0M;1.0M|],typeof<MyTuple>)
let myTuple = ret :?> MyTuple

getting myTuple of type MyTuple , where the latter carries type information about tuple consituents at compile time .