hubFS: THE place for F#

Hi,

I'm trying to implement a simple TcpChannel between 2 AppDomains and need to pass an object (by value) from one to another. The object is a SerializableDictionary but also needs a default constructor and a constructor of type:

protected SerializableDictionary(SerializationInfo info, StreamingContext context) : base(info, context) { }

I can't seem to be able to get both constructors into an F# type.

open System
open System.Collections.Generic
open System.Xml
open System.Xml.Serialization
open System.Runtime.Serialization

[<Serializable>]

[<XmlRoot("dictionary")>]

type SerializableDictionary<'TKey, 'TValue>(info: SerializationInfo, context: StreamingContext) =
    inherit Dictionary<'TKey, 'TValue>(info, context)
    new() = SerializableDictionary<'TKey, 'TValue>(null, new StreamingContext())

causes a run-time exception:

ERROR: Object reference not set to an instance of an object.

I believe that F# requires all contructors to call the 'primary' contructor, but this doesn't seem correct here. Is it possible for an F# type to contain these two constructors?

All help much appreciated,

Danny

If you want two different constructors in a derived class to call into two different constructors in a base class, then you must use the 'explicit' class syntax, where there are no parens after the type/inherit declaration, and instance variables use 'val'.

http://msdn.microsoft.com/en-us/library/dd469494(VS.100).aspx

Here's an example:

[<Serializable>]
[<XmlRoot("dictionary")>]
type SerializableDictionary<'TKey, 'TValue> =
    inherit Dictionary<'TKey, 'TValue>
    val anotherInstanceVar : int
    new(info: SerializationInfo, context: StreamingContext) = {inherit Dictionary<'TKey, 'TValue>(info,context); anotherInstanceVar = 4}
    new() = {inherit Dictionary<'TKey, 'TValue>(); anotherInstanceVar = 0}

This is one of the very few cases where you need to use this 'explicit' syntax rather than the preferred syntax involving a primary constructor.

Thanks Brian - that works nicely.

regards,

Danny