Deal with versionless cards

[paulcam206]

In the past we allowed cards with no explicit set version to parse (I believe we implied a version of 0.5). That was removed as is was thought to no longer be needed. We need to revisit that decision

[matthidinger]

I think the last time Gilles and I talked about this we were considering a MissingVersion callback API or similar. I can't recall if we just ran out of time to implement it or found flaws in the design.

Essentially it was something like

// AdaptiveCard.cs

public static Func<AdaptiveVersion> OnDeserializingMissingVersion { get; set; }

// then later where we deserialize and check for version

if(version == null) 
{
   if(OnDeserializingMissingVersion == null)
       throw new AdaptiveException("Missing required property 'version'");
   else
       card.Version = OnDeserializingMissingVersion();
}

The caller (BF) would use like:

AdaptiveCard.OnDeserializeMissingVersion = () => new AdaptiveVersion(0, 5);
AdaptiveCard.FromJson("....");

[shalinijoshi19]

Thanks Matt for the context. Was just reading up on the custom converters, and it almost looks like we can get away with having a private constructor that takes in a boolean around whether or not we want to be strict with version checks; And then BF can call it with the ConstructorHandling.AllowNonPublicDefaultConstructor setting to make us set that flag when they call us. Maybe? @paulcam206, something to look into as you investigate. Matt brings up a good point around all of this being reflection driven code executed by Newtonsoft so limited flexibility but still some. Good to jot down some options, and float around a proposal please for discussion/review!

[shalinijoshi19]

@paulcam206 update on this? Would this be ready by 9/18's bugbash event?