Register any TCollection type for proper JSON serialization

Due to the current implementation pattern of the TCollection type in Delphi, it was not possible to implement directly this kind of parameter.

In fact, the TCollection constructor is defined as such:

 constructor Create(ItemClass: TCollectionItemClass);

And, on the server side, we do not know which kind of TCollectionItemClass is to be passed. Therefore, the TServiceFactoryServer is unable to properly instantiate the object instances, supplying the expected item class.

The framework propose two potential solutions:

You can let your collection class inherit from the new TInterfaceCollection type;
You can call the TJSONSerializer.RegisterCollectionForJSON() method to register the collection type and its associated item class.

First solution has already been detailed in this blog.
We will now describe the second (and new) way.

The other way of using TCollection kind of parameters is to declare it explicitly to the framework. You should call JSONSerializer.RegisterCollectionForJSON() with the corresponding TCollection / TCollectionItem class type pair.

Consider a dedicated class:

 TMyCollection = type(TCollection)

Note that a dedicated type is needed here. You just can't use this registration over a plain TCollection.

Then, for instance, after calling:

 TJSONSerializer.RegisterCollectionForJSON(TMyCollection,TMyCollectionItem);

The following lines of code are the same:

 MyColl := TMyCollection.Create(TMyCollection);
 MyColl := ClassInstanceCreate(TMyCollection) as TMyCollection;
 MyColl := ClassInstanceCreate('TMyCollection') as TMyCollection;

Thanks to this internal registration table, mORMot will be able to serialize and unserialize plain TCollection type, without the need of inheriting from a dedicated type.

Feedback is welcome on our forum.

Synopse Open Source

Register any TCollection type for proper JSON serialization