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ISerializable and constructed classes
f#classes

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:

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

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

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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'.

[link:msdn.microsoft.com]

Here's an example:

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[<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.

By on 7/2/2009 12:47 PM ()

Thanks Brian - that works nicely.

regards,

Danny

By on 7/3/2009 8:46 AM ()

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