I have an interface in C# like this:
public interface IMessageOptions
{
int ReceiveTimeout { get; set; }
int PeekTimeout { get; set; }
bool Transactional { get; set; }
}
and I'm trying to implement it in F# like this:
type Settings() =
interface IMessageOptions with
member this.PeekTimeout: int with get, set
member this.ReceiveTimeout: int with get, set // error at start of this line
member this.Transactional: bool with get, set
But I get an error after the first property saying:
"Incomplete structured construct at or before this point in pattern"
How should this be written?
Are auto properties not allowed in this context?
Here is the syntax for that:
type Settings() =
interface IMessageOptions with
member val PeekTimeout = 0 with get, set
member val ReceiveTimeout = 0 with get, set
member val Transactional = false with get, set
The differences are:
Use val instead of this.
Define the default values explicitly
You don't have to specify the types (although you can)
For the reference, here is how you would define the interface itself in F#:
type IMessageOptions =
abstract member PeekTimeout : int with get, set
abstract member ReceiveTimeout : int with get, set
abstract member Transactional : bool with get, set
If you want automatically implemented properties, then this is the syntax:
type Settings() =
interface IMessageOptions with
member val PeekTimeout : int = 0 with get, set
member val ReceiveTimeout : int = 0 with get, set
member val Transactional : bool = false with get, set
Notice that you explicitly have to define the default value.
Related
I need to implement an interface like this:
interface IEvent<T> : IEvent {
T Data { get; }
}
public interface IEvent
{
Guid Id { get; set; }
long Version { get; set; }
long Sequence { get; set; }
object Data { get; }
Guid StreamId { get; set; }
string StreamKey { get; set; }
DateTimeOffset Timestamp { get; set; }
string TenantId { get; set; }
Type EventType { get; }
string EventTypeName { get; set; }
string DotNetTypeName { get; set; }
}
That's what I came up with:
type WrappedEvent<'T>(x: 'T) =
interface Events.IEvent with
member val Data = x with get
member val DotNetTypeName = null with get, set
member val EventType = null
member val EventTypeName = null with get, set
member val Id = Guid.Empty with get, set
member val Sequence = int64 (0) with get, set
member val StreamId = Guid.Empty with get, set
member val StreamKey = null with get, set
member val TenantId = null with get, set
member val Timestamp = DateTimeOffset.MinValue with get, set
member val Version = int64 (0) with get, set
WrappedEvent is used like this:
let MapToSubtype subtype =
match subtype with
| CustomerRegistered registeredCustomer -> WrappedEvent<CustomerRegisteredEvent> registeredCustomer :> Events.IEvent
| CustomerDeleted deletedCustomer -> WrappedEvent<CustomerDeletedEvent> deletedCustomer :> Events.IEvent
The compiler throws an error for <'T> at WrappedEvent<'T>
This type parameter has been used in a way that constrains it to always be 'obj'
This code is less generic than required by its annotations because the explicit type variable 'T' could not be generalized.
It was constrained to be 'obj'.
A warning is shown for Data at member val Data:
This construct causes code to be less generic than indicated by the type annotations.
The type variable 'T has been constrained to be type 'obj'.
How do I solve this?
Update:
If I'm implementing the interface like this interface Events.IEvent<'T> with (using the generic parameter), I get this error:
No implementation was given for 'Events.IEvent.get_Data() : obj'.
Note that all interface members must be implemented and listed under an appropriate 'interface' declaration, e.g. 'interface ... with member ...'.
First, you're implementing two interfaces here, not one, - IEvent<T> and its base interface IEvent. As such, you need to add two interface ... with blocks - one for each:
type WrappedEvent<'T>(x: 'T) =
interface Events.IEvent<'T> with
member val Data = x with get
interface Events.IEvent with
member val Data = x with get
member val DotNetTypeName = null with get, set
member val EventType = null
...
Second, when implementing the IEvent interface, note the type of its Data member: it's obj. So if you initialize that member with x, it follows that x must have type obj. And that's the source of your original error: x is used in such a way that will make it constrained to obj.
To fix this, you need to downcast x to obj before initializing Data with it:
interface Events.IEvent with
member val Data = x :> obj with get
But wait! This will still not work. The problem is, member val declares not just a property, but also a backing field for it. And now you're declaring two different backing fields, both named Data, but of different types. (also, btw, you don't actually need separate backing fields, right? because you already have x)
So to get around that, make them properties without backing fields:
interface Events.IEvent<'T> with
member self.Data with get() = x
interface Events.IEvent with
member self.Data with get() = x :> obj
...
How to translate the following COM interface to F#? I cannot figure out how to annotate get and set of a property.
Plus, for COM interop, do I need to annotate both a property itself and its get with DispId?
[ComImport, TypeLibType((short)0x1040), Guid("F935DC23-1CF0-11D0-ADB9-00C04FD58A0B")]
private interface IWshShortcut
{
[DispId(0)]
string FullName { [return: MarshalAs(UnmanagedType.BStr)] [DispId(0)] get; }
[DispId(0x3e8)]
string Arguments { [return: MarshalAs(UnmanagedType.BStr)] [DispId(0x3e8)] get; [param: In, MarshalAs(UnmanagedType.BStr)] [DispId(0x3e8)] set; }
[DispId(0x3ec)]
string RelativePath { [param: In, MarshalAs(UnmanagedType.BStr)] [DispId(0x3ec)] set; }
[DispId(0x3ee)]
int WindowStyle { [DispId(0x3ee)] get; [param: In] [DispId(0x3ee)] set; }
[DispId(0x3ef)]
string WorkingDirectory { [return: MarshalAs(UnmanagedType.BStr)] [DispId(0x3ef)] get; [param: In, MarshalAs(UnmanagedType.BStr)] [DispId(0x3ef)] set; }
[TypeLibFunc((short)0x40), DispId(0x7d0)]
void Load([In, MarshalAs(UnmanagedType.BStr)] string PathLink);
[DispId(0x7d1)]
void Save();
}
Here's a correct, though not literal, translation:
[<ComImport; Guid("F935DC23-1CF0-11D0-ADB9-00C04FD58A0B"); TypeLibType(0x1040s)>]
type private IWshShortcut =
[<DispId(0)>]
abstract member FullName : [<MarshalAs(UnmanagedType.BStr)>] string with get
[<DispId(0x3e8)>]
abstract member Arguments : [<MarshalAs(UnmanagedType.BStr)>] string with get, set
[<DispId(0x3ec)>]
abstract member RelativePath : [<MarshalAs(UnmanagedType.BStr)>] string with set
[<DispId(0x3ee)>]
abstract member WindowStyle : int with get, set
[<DispId(0x3ef)>]
abstract member WorkingDirectory : [<MarshalAs(UnmanagedType.BStr)>] string with get, set
[<DispId(0x7d0); TypeLibFunc(0x40s)>]
abstract member Load : [<MarshalAs(UnmanagedType.BStr)>] PathLink:string -> unit
[<DispId(0x7d1)>]
abstract member Save : unit -> unit
As you found, you cannot add attributes to an abstract property's underlying getter/setter methods in F#, only to the property itself, but it doesn't matter for this particular interface:
String properties with both get and set need the same MarshalAs for both anyway.
In is the default directionality for string parameters, so specifying it would be redundant anyway.
Applying DispId to the property getter as your C# code does is legal but pointless – while DispId can be applied to both methods and properties, and property getters and setters technically happen to be methods, the attribute only has an effect for the property itself.
N.b. because the CLR marshals string parameters for COM methods as BStrs by default, we can omit all the MarshalAs directives as well and make this look a bit more trim (albeit less explicit):
[<ComImport; Guid("F935DC23-1CF0-11D0-ADB9-00C04FD58A0B"); TypeLibType(0x1040s)>]
type private IWshShortcut =
[<DispId(0)>] abstract member FullName:string with get
[<DispId(0x3e8)>] abstract member Arguments:string with get, set
[<DispId(0x3ec)>] abstract member RelativePath:string with set
[<DispId(0x3ee)>] abstract member WindowStyle:int with get, set
[<DispId(0x3ef)>] abstract member WorkingDirectory:string with get, set
[<DispId(0x7d0); TypeLibFunc(0x40s)>] abstract member Load : PathLink:string -> unit
[<DispId(0x7d1)>] abstract member Save : unit -> unit
Of course, all of this applies to the C# implementation as well, so it can be similarly simplified:
[ComImport, Guid("F935DC23-1CF0-11D0-ADB9-00C04FD58A0B"), TypeLibType((short)0x1040)]
private interface IWshShortcut
{
[DispId(0)] string FullName { get; }
[DispId(0x3e8)] string Arguments { get; set; }
[DispId(0x3ec)] string RelativePath { set; }
[DispId(0x3ee)] int WindowStyle { get; set; }
[DispId(0x3ef)] string WorkingDirectory { get; set; }
[DispId(0x7d0), TypeLibFunc((short)0x40)] void Load(string PathLink);
[DispId(0x7d1)] void Save();
}
I am having difficulty to convert following C# code to F#:
class Foo
{
public Foo() { }
public Foo(string name) { }
}
class Bar : Foo
{
public Bar() : base() { }
public Bar(string name) : base(name) { }
public string Name { get; set; }
}
I first tried following, but it is reporting error
Constructors for the type 'Bar' must directly or indirectly call its
implicit object constructor. Use a call to the implicit object
constructor instead of a record expression.
type Foo() =
new(name:string) = Foo()
type Bar() =
inherit Foo()
new(name:string) = { inherit Foo(name) }
member val Name:string = null with get, set
Then I tried following, but it is now reporting error on the auto property
'member val' definitions are only permitted in types with a primary
constructor. Consider adding arguments to your type definition"
type Foo() =
new(name:string) = Foo()
type Bar =
inherit Foo
new(name:string) = { inherit Foo(name) }
member val Name:string = null with get, set
If you want F# source code who compiles to precisely the same API as given by your C# code, the answer is as follows:
type Foo =
new() = {}
new(name:string) = { }
type Bar =
inherit Foo
[<DefaultValue>]
val mutable private name:string
new() = { inherit Foo() }
new(name) = { inherit Foo(name) }
member x.Name with get() = x.name and set v = x.name <- v
This compiles:
type Foo() =
new(name:string) = Foo()
type Bar(name : string) =
inherit Foo()
new() = Bar(null) // or whatever you want as a default.
member val Name:string = name with get, set
See Constructors (F#) and Inheritance (F#).
Looking at the decompilation, the C# would be (with attributes removed):
public class Bar : Program.Foo {
internal string Name#;
public string Name {
get {
return this.Name#;
}
set {
this.Name# = value;
}
}
public Bar(string name) {
this.Name# = name;
}
public Bar() : this(null) {
}
}
public class Foo {
public Foo() {
}
public Foo(string name) : this() {
}
}
If a class has a parameter list directly after its name (including ()), it has a primary constructor. Using it, any inherit declarations are placed only in this primary constructor, which comes directly after the class declaration and before any member declarations.
It is unclear what you are trying to achieve. The class Foo has a constructor taking a string argument, only to discard it. A (technically) valid, similar pair of classes would be this:
type Foo(name:string) =
member f.NameLength = name.Length
type Bar(initialName) = // WARNING: this will not end well
inherit Foo(initialName)
member val Name:string = initialName with get, set
But this is not sensible code. Foo will keep the initial name even if the name in Bar is changed. Bar.Name.Length returns the current name's length, while Bar.NameLength returns the initial name's length.
To keep the default constructor, one could add new () = Bar(null) (or the equivalent in Foo), but please note that null is considered an interop-only feature. It is not used in F# facing code; if possible, use the appropriate option type or an empty string respectively (depending on whether the string is just empty or doesn't exist at all).
Also, inheriting classes is discouraged in the F# component design guidelines -- for good reason. There are few use cases, but those usually involve a tiny base class and a derived class that is a perfect superset of it. It is far more common to compose types by using one class as a member of another.
I don't know how relevant this is, but here is an example of a class with default constructor and an additional constructor that uses it:
type Text500(text : string) =
do if text.Length > 500 then
invalidArg "text" "Text of this type cannot have a length above 500."
member t.Text = text
new () = Text500("")
This utilizes the primary constructor to verify input and has an additional, parameterless constructor that uses an empty string. (I'm not sure if the additional constructor would be useful in actual applications.)
I did not find a clear answer for this...
What is the F# equivalent of this C# code:
public class SomeClass
{
public virtual SomeMethod([Attribute] Int32 param)
{ }
}
Because there are two different places to put the attribute in F#:
type SomeClass () =
abstract SomeMethod : [<Attribute>] param:Int32 -> Unit
default this.SomeMethod ([<Attribute>] param) = ()
You can put it on both, but the override (default) is the one that counts.
type T =
abstract M : (* [<Out>] this is okay too *) i: byref<int> -> unit
default this.M([<Out>] i) = () //but this one's necessary
Hey there! I'm trying to write a POCO class in proper F#... But something is wrong..
The C# code that I want to "translate" to proper F# is:
public class MyTest
{
[Key]
public int ID { get; set; }
public string Name { get; set; }
}
The closest I can come to the above code in F# is something like:
type Mytest() =
let mutable _id : int = 0;
let mutable _name : string = null;
[<KeyAttribute>]
member x.ID
with public get() : int = _id
and public set(value) = _id <- value
member x.Name
with public get() : string = _name
and public set value = _name <- value
However when I try to access the properties of the F# version it just returns a compile error saying
"Lookup on object of indeterminate type based on information prior to this program point. A type annotation may be needed prior to this program point to constrain the type of the object. This may allow the lookup to be resolved."
The code thats trying to get the property is a part of my Repository (I'm using EF Code First).
module Databasethings =
let GetEntries =
let ctx = new SevenContext()
let mydbset = ctx.Set<MyTest>()
let entries = mydbset.Select(fun item -> item.Name).ToList() // This line comes up with a compile error at "item.Name" (the compile error is written above)
entries
What the hell is going on?
Thanks in advance!
Your class definition is fine, it's your LINQ that has a problem. The Select method is expecting an argument of type Expression<Func<MyTest,T>> but you're passing it a value of type FSharpFunc<MyTest,T> - or something similar to that anyway.
The point is you can't use F# lambda expressions directly with LINQ. You need to write your expression as an F# Quotation and then use the F# PowerPack to run the code against an IQueryable<> data source. Don Syme has a good overview of how this works.