F# Reflection Robust reflection using F# quotations
Example
Reflection is useful but fragile. Consider this:
let mi = typeof<System.String>.GetMethod "StartsWith"
The problems with this kind of code are:
- The code doesn't work because there are several overloads of
String.StartsWith - Even if there wouldn't be any overloads right now later versions of the library might add an overload causing a runtime crash
- Refactoring tools like
Rename methodsis broken with reflection.
This means we get a runtime crashes for something that is known compile-time. That seems suboptimal.
Using F# quotations it's possible to avoid all the problems above. We define some helper functions:
open FSharp.Quotations
open System.Reflection
let getConstructorInfo (e : Expr<'T>) : ConstructorInfo =
match e with
| Patterns.NewObject (ci, _) -> ci
| _ -> failwithf "Expression has the wrong shape, expected NewObject (_, _) instead got: %A" e
let getMethodInfo (e : Expr<'T>) : MethodInfo =
match e with
| Patterns.Call (_, mi, _) -> mi
| _ -> failwithf "Expression has the wrong shape, expected Call (_, _, _) instead got: %A" e
We use the functions like this:
printfn "%A" <| getMethodInfo <@ "".StartsWith "" @>
printfn "%A" <| getMethodInfo <@ List.singleton 1 @>
printfn "%A" <| getConstructorInfo <@ System.String [||] @>
This prints:
Boolean StartsWith(System.String)
Void .ctor(Char[])
Microsoft.FSharp.Collections.FSharpList`1[System.Int32] Singleton[Int32](Int32)
<@ ... @> means that instead of executing the expression inside F# generates an expression tree representing the expression.
<@ "".StartsWith "" @> generates an expression tree that looks like this: Call (Some (Value ("")), StartsWith, [Value ("")]).
This expression tree matches what getMethodInfo expects and it will return the correct method info.
This resolves all the problems listed above.
