I have the following C# method:
private static bool IsLink(string shortcutFilename)
{
var pathOnly = Path.GetDirectoryName(shortcutFilename);
var filenameOnly = Path.GetFileName(shortcutFilename);
var shell = new Shell32.Shell();
var folder = shell.NameSpace(pathOnly);
var folderItem = folder.ParseName(filenameOnly);
return folderItem != null && folderItem.IsLink;
}
I have tried converting this to F# as:
let private isLink filename =
let pathOnly = Path.GetDirectoryName(filename)
let filenameOnly = Path.GetFileName(filename)
let shell = new Shell32.Shell()
let folder = shell.NameSpace(pathOnly)
let folderItem = folder.ParseName(filenameOnly)
folderItem <> null && folderItem.IsLink
It however reports an error for the let shell = new Shell32.Shell() line, saying that new cannot be used on interface types.
Have I just made a silly syntactic mistake, or is there extra work needed to access COM from F#?
I don't know enough about the F# compiler but your comments makes it obvious enough. The C# and VB.NET compilers have a fair amount of explicit support for COM built-in. Note that your statement uses the new operator on an interface type, Shell32.Shell in the interop library looks like this:
[ComImport]
[Guid("286E6F1B-7113-4355-9562-96B7E9D64C54")]
[CoClass(typeof(ShellClass))]
public interface Shell : IShellDispatch6 {}
IShellDispatch6 is the real interface type, you can also see the IShellDispatch through IShellDispatch5 interfaces. That's versioning across the past 20 years at work, COM interface definitions are immutable since changing them almost always causes an undiagnosable hard crash at runtime.
The [CoClass] attribute is the important one for this story, that's what the C# compiler goes looking for you use new on a [ComImport] interface type. Tells it to create the object by creating an instance of Shell32.ShellClass instance and obtain the Shell interface. What the F# compiler doesn't do.
ShellClass is a fake class, it is auto-generated by the type library importer. COM never exposes concrete classes, it uses a hyper-pure interface-based programming paradigm. Objects are always created by an object factory, CoCreateInstance() is the workhorse for that. Itself a convenience function, the real work is done by the universal IClassFactory interface, hyper-pure style. Every COM coclass implements its CreateInstance() method.
The type library importer makes ShellClass look like this:
[ComImport]
[TypeLibType(TypeLibTypeFlags.FCanCreate)]
[ClassInterface(ClassInterfaceType.None)]
[Guid("13709620-C279-11CE-A49E-444553540000")]
public class ShellClass : IShellDispatch6, Shell {
// Methods
[MethodImpl(MethodImplOptions.InternalCall, MethodCodeType=MethodCodeType.Runtime), DispId(0x60040000)]
public virtual extern void AddToRecent([In, MarshalAs(UnmanagedType.Struct)] object varFile, [In, Optional, MarshalAs(UnmanagedType.BStr)] string bstrCategory);
// Etc, many more methods...
}
Lots of fire and movement, none of it should ever be used. The only thing that really matters is the [Guid] attribute, that provides the CLSID that CoCreateInstance() needs. It also needs the IID, the [Guid] of the interface, provided by the interface declaration.
So the workaround in F# is to create the Shell32.ShellClass object, just like the C# compiler does implicitly. While technically you can keep the reference in a ShellClass variable, you should strongly favor the interface type instead. The COM way, the pure way, it avoids this kind of problem. Ultimately it is the CLR that gets the job done, it recognizes the [ClassInterface] attribute on the ShellClass class declaration in its new operator implementation. The more explicit way in .NET is to use Type.GetTypeFromCLSID() and Activator.CreateInstance(), handy when you only have the Guid of the coclass.
Related
I'm making a project where there are separate source files/modules that add functions to a single Dictionary contained in a higher level file. However, I find that nothing in these source files evaluates on its own, even functions that take no arguments/code that isn't even inside a function.
As a result nothing is being added to the Dictionary. Is there a way to forcibly evaluate complete function calls in a module automatically? I'll give an example of what I'm trying to do:
Registry.fs:
let private functions = Dictionary<string, MessageHandler>()
let add type handler =
functions.Add(type, handler)
Handler1.fs:
Registry.add "type1" (fun m -> ....
)
Handler2.fs:
Registry.add "type2" (fun m -> ....
)
I believe you need to see this relevant topic. Loose method calls would get compiled as method calls inside of a static constructor for the enclosing type/module, when the F# code gets compiled to IL. This would roughly be equivalent to the following C# code, just to see the picture:
static class Handler1 {
static Handler1() {
// this is the static constructor
Registry.add "type1" ....
}
}
In .NET static constructors are not eagerly initialized1. This means, if you want to cause the .NET runtime to call the Handler1 static constructor, you need to access a static member of the type Handler1.
An example of using the type in a static context would be to
Expose a sufficiently accessible static member/method:
module Handler1 =
[<Literal>]
let Name = "Handler1"
Access that static member from your code, such as the main method:
[<EntryPoint>]
let main args =
printf Handler1.Name
The above line will force the .NET runtime to load the Handler1 type's static context, which will result in invoking the static constructor if the type is encoutered by your code for the first time. If your code never encounters a given type's static context (any static member or method), then it will never be initialized -- the static constructors will never get called.
This behaviour is by design of the .NET framework (and that is regardless of the chosen language -- C#, F#, VB, others -- they all compile to similar IL). The point is to not allocate unnecessary resources by types that are never actually used.
1 Until .NET 4, static type context was initialized when the given type was first encountered by the executing code, regardless if the user code is interacting with instace or static members of that type. After .NET 4, this slightly changed -- the static context is initialized only when the user code interacts with static members of the type.
I am trying to get this example translated from C# to F#
public class MyModule : NancyModule
{
private IMyDependency _dependency;
public MyModule(IMyDependency dependency)
{
_dependency = dependency;
Get["/"] = x =>
{
};
// Register other routes
}
}
(source 1)
However adding a parameter to constructor
type HelloModule(dependency) as self =
inherit NancyModule()
do
self.Get.["/"] <- fun _ -> "Hello" :> obj
(source 2)
results in a run-time exception: System.InvalidOperationException: 'Something went wrong when trying to satisfy one of the dependencies during composition ...
How can I correctly add a dependency like a data-source to the code? Or, generally, how do I pass something from outside of HelloModule to the inside?
I'm guessing this might be caused by not specifying the type of the dependency parameter of the constructor in your F# code. This would result in the F# compiler assigning that parameter a generic type, and then Nancy's dependency injection framework doesn't know what to inject.
Try the following and see if it fixes your problem:
type HelloModule(dependency : IMyDependency) as self =
inherit NancyModule()
do
self.Get.["/"] <- fun _ -> "Hello" :> obj
P.S. Naturally, for this to work, you'll also need to have some type that implements the IMyDependency interface, and have told the Nancy framework about that type. From this part of the Nancy documentation that you linked to, it looks like merely declaring the type is enough, but if that's not actually enough then you'll have to register the type manually. I'm not familiar enough with Nancy to give you specific advice there beyond what the documentation says.
Just curious why F# has:
member val Foo = ... with get, set
While omitting the self identifier (e.g. this.).
This is still an instance property. Maybe I am the only one confused when using it. But just bothered me enough to query whoever knows how the language was defined.
With this syntax, the property is almost totally auto-implemented -- all you provide is the initialization code, which essentially runs as part of the constructor.
One of the best-practice guard rails F# puts in place is that it does not let you access instance members before the instance is fully initialized. (wow, crazy idea, right?).
So you would have no use for a self-identifier in auto-props, anyways, since the only code you get to write is init code that can't touch instance members.
Per the MSDN docs (emphasis mine):
Automatically implemented properties are part of the initialization of
a type, so they must be included before any other member definitions,
just like let bindings and do bindings in a type definition. Note that
the expression that initializes an automatically implemented property
is only evaluated upon initialization, and not every time the property
is accessed. This behavior is in contrast to the behavior of an
explicitly implemented property. What this effectively means is that
the code to initialize these properties is added to the constructor of
a class.
Btw, if you try to be a smartass and use the class-level self-identifier to get around this, you'll still blow up at runtime:
type A() as this =
member val X =
this.Y + 10
with get, set
member this.Y = 42
let a = A()
System.InvalidOperationException: The initialization of an object or value resulted in an object or value being accessed recursively before it was fully initialized.
at Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicFunctions.FailInit()
at FSI_0013.A.get_Y()
at FSI_0013.A..ctor()
at <StartupCode$FSI_0014>.$FSI_0014.main#()
Edit: Worth noting that in upcoming C# 6, they also now allow auto-props with initializers (more F# features stolen for C#, shocker :-P), and there is a similar restriction that you can't use the self-identifier:
class A
{
// error CS0027: Keyword 'this' is not available in the current context
public int X { get; set; } = this.Y + 10;
public int Y = 42;
public A() { }
}
I have a file with a module with some routines that take parameters and return unit, these routines have side-effects. I noticed that when accessing these f# routines from c# they're actually properties of type unit and when I try to access 1 property, it runs all properties in the module.
From the F# documentation all top level do bindings are run on type initialization.
What is the preferred way to write functions that should not be run on type initialization but are also not associated with other state i.e. a class with functions and member variables?
Should I put these functions inside a type and just have no records in the type?
Code example:
namespace test_space
open System.Diagnostics;
module test =
let test_1 =
Debug.WriteLine ("One")
let test_2 =
Debug.WriteLine ("Two")
I'm running this code with C#:
static void Main (string [] args)
{
Object o;
o = test.test_2;
}
And the output is:
One
Two
The problem is you didn't create functions but value bindings. test_1 is a value. test_1() is a function of type unit -> unit. Make sure you put () after the function name.
I don't fully understand the scenario you're describing - F# functions declared in a module will generally appear as methods and values will appear as properties. The code that is executed when you first access module (type initialization) is the initialization of values.
If you write just:
module Foo =
let Operation () =
printfn "hello"
...then calling Operation will be a method and calling Foo.Operation() will run the side-effect. If you can post some code that behaves unexpectedly, then someone can explain it.
Anyway, if you want to be sure about the behavior, you can write operations as static members of a class:
type Foo =
static member Operation() =
printfn "hello"
Then you can be sure that F# will compile them as static members of a class in a predictable way.
I've been struggling to get this to compile for about an hour. It must be something stupid. Can you spot it?
in my lib project:
namespace TravelerStuff
open System
type Traveler =
abstract GetData : unit -> unit
type public DeltaTraveler() =
interface Traveler with
member v.GetData () =
printf "hello"
and in my console test app:
[<EntryPoint>] let main _ =
let traveler = new TravelerStuff.DeltaTraveler()
traveler.GetData // this line won't compile: (The field, constructor or member 'GetData' is not defined)
As gradbot says, F# doesn't currently implicitly convert values to interfaces when searching for members. Also, F# only uses explicit interface implementation (as known from C#) and not implicit implementation where members are not only compiled as implementation of an interface, but also as ordinary (directly visible) members of the type.
Aside from casting, you can duplicate the member in the type definition:
type DeltaTraveler() =
member v.GetData () = printf "hello"
interface Traveler with
member v.GetData () = v.GetData()
Also, if you just want to implement an interface, but don't need to add any members, you can use F# object expressions (which are more lightweight):
let deltaTraveler() =
{ new Traveler with
member v.GetData () = printf "hello" }
// The function directly returns value of type 'Traveler'
let t = deltaTraveler()
t.GetData()
You need to upcast. F# currently won't do it for you in this situation.
(traveler :> TravelerStuff.Traveler).GetData()
// open the namespace to reduce typing.
open TravelerStuff
(traveler :> Traveler).GetData()
Snip from F# docs.
In many object-oriented languages,
upcasting is implicit; in F#, the
rules are slightly different.
Upcasting is applied automatically
when you pass arguments to methods on
an object type. However, for let-bound
functions in a module, upcasting is
not automatic, unless the parameter
type is declared as a flexible type.
For more information, see Flexible Types (F#).