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Proper syntax for passing 'this' in a constructor?
f#

What is the proper syntax for passing 'self' in a constructor?

I have the following C# library code:

public abstract class CollectionKind
{
//...
}

public class BoolKind : IBoolKind
{
public BoolKind(CollectionKind parent)
{
//...
}
//...
}

What is the equivalant F# syntax of the following?

public class DerivedCollection : CollectionKind
{
public DerivedCollectionKind()
{
isActive = new BoolKind(this);
}

public IBoolKind IsActive { get { return isActive; } }
private readonly BoolKind isActive;
}

My main problem is finding the correct syntax for initializing isActive while passing 'this'.

On another note, is there any good guide for trans-literating C# to F#? I have a bunch of legacy code that I would like to begin moving over to F# and would like to keep it "as is" for now. I don't want to change languages and rewrite the code in a single step.

.

Hi,
you can write the following:

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type DerivedCollection() as this =
  inherit CollectionKind()
  let isActive = new BoolKind(this)
  member x.IsActive = isActive
By on 11/14/2008 8:27 AM ()

Thanks for the quick reply. I tried your suggestion and I get a blue squiggly over the 'this' in BoolKind(this) with the following error:

Recursive references to the object being defined will be checked for initialization soundness at runtime through the use of a delayed reference. Consider placing self-references within 'do' statements after the last 'let' binding in the construction sequence.

When I run I get a null reference exception on the same line.

Any ideas? - Jake

By on 11/14/2008 9:01 AM ()

Ahh, sorry!
The problem was that the code used 'this' before the class was fully initialized. This could lead to errors (where some fields would have 'null' values), so F# tries to prevent that - and shows the warning you've seen.

To workaround this, you can use mutable values and initialize them once the class is constructed. You can use 'option' type during the initialization to make sure that all possible errors will be handled properly:

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type CollectionKind() =
  member x.A = "ZZ"

type BoolKind(c:CollectionKind) =
  do printfn "%s" c.A
        
type DerivedCollection() as this =
  inherit CollectionKind()
  let mutable isActive = None
  do 
    isActive <- Some(BoolKind(this))
  member x.IsActive = match isActive with | Some(v) -> v | _ -> failwith "Wrong initialization!"

An alternative "unsafe" way is to use the DefaultValue attribute:

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type DerivedCollection() as this =
  inherit CollectionKind()
  
  // Value will be 'null' until the class is constructed
  [<DefaultValue(false)>]
  val mutable isActive : BoolKind 
  do 
    this.isActive <- new BoolKind(this)
    
  member x.IsActive = this.isActive

T.

By on 11/14/2008 9:20 AM ()

Works. Thanks!

By on 11/14/2008 12:18 PM ()

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