Multiple Constructors in f# with property assignment

Question

I want to have a constructor that is blank and a constructor overload that accepts a parameter and assign it to a public property.

This is where I am stuck:

type TemplateService() = 
 interface ITemplateService with
 //Properties
 member TemplateDirectory = ""
 //Constructors
 new (templateDirectory:string) = //Error here.
 if (templateDirectory == null) then
 raise (new System.ArgumentNullException("templateDirectory"))
 TemplateDirectory = templateDirectory;

It gives me the error: `Unexpected keyword 'new' in the Object expression. Expected 'member', 'override' or other token.

If I use member, the property TemplateDirectory gives this error:

This instance member needs a parameter to represent the object being invoked. Make the member static or use the notation 'member x.Member(args) = ...'

Answer 1

You could try this.

type TemplateService(templateDirectory : string) = 
 do
 if templateDirectory = null then nullArg "templateDirectory"
 new() = TemplateService("")
 interface ITemplateService with
 member this.TemplateDirectory = templateDirectory

• Shawn Mclean: what if I wanted to check if d is null or empty before assigning? Such as throwing an error before it.
• Shawn Mclean: But I only need to do this test for the constructor that accepts an arguement. If I do new TemplateService(), then it will throw an error.
• Nyi Nyi: I don't think the above code snippet will throw exception if you do new TemplateService(). It will be good if i can see your actual code.

Answer 2

Unfortunately if you want to use an interface and pass the value to the constructor nyinyithann answer is correct. You can set public properties in a constructor call like this.

type TemplateService() = 
 let mutable templateDirectory = ""
 member this.TemplateDirectory
 with get() = templateDirectory
 and set directory = 
 if directory = null then
 raise (new System.ArgumentNullException "templateDirectory")
 templateDirectory <- directory
let template = TemplateService(TemplateDirectory = "root")

Now if you want to use an interface this won't work.

type ITemplateService =
 abstract TemplateDirectory : string with get, set
type TemplateService() = 
 let mutable templateDirectory = ""
 interface ITemplateService with
 member this.TemplateDirectory
 with get() = templateDirectory
 and set directory = 
 if directory = null then
 raise (new System.ArgumentNullException "templateDirectory")
 templateDirectory <- directory
let template = TemplateService(TemplateDirectory = "root") // error

You're forced to use this ugly thing.

let template = TemplateService()
(template :> ITemplateService).TemplateDirectory <- "root"

• Guvante: My recommendation when you want interface and normal access is to simply define it twice, once outside the interface, and a wrapper within the interface.

Answer 3

You are putting your constructor in the definition of the interface, and that is the cause for the error. Also you are attempting to store a value to a get only property, instead you should use a backing store.

Finally I would recommend nyinyithann's version over this, since it is more inline with the usual F# style (minimal mutables), just hoping to give a version closer to yours in case it is helpful.

type TemplateService() = 
 let mutable directory = ""
 interface ITemplateService with
 //Properties
 member this.TemplateDirectory = directory
 //Constructors
 new (templateDirectory:string) =
 if (templateDirectory = null) then
 raise (new System.ArgumentNullException("templateDirectory"))
 directory <- templateDirectory;