I don't know if i will prefer override the %JSONExportToString and %JSONExportToStream or  wrapping but that dirty trick..
 

Class test.elementList Extends (%ListOfObjects, %JSON.Adaptor)
{



/// The type (class name) of the elements stored in the collection.

Parameter ELEMENTTYPE = "test.element";



/// Returns this object as a JSON Stream
Method %JSONExportToStream(ByRef objStream As %Stream.Object, %mappingName As %String = "") As %Status
{

    #Dim objStream As %Stream.TmpCharacter = ##class(%Stream.TmpCharacter).%New()

    Do objStream.Write("[")

    For i=1:1:..Size
    {

        Do ..GetAt(i).%JSONExportToStream(.objStream, %mappingName)

        If (i<..Size)
        {

            Do objStream.Write(",")
        }
    }

    Do objStream.Write("]")

    Return $$$OK
}



/// Returns this object as a JSON string
Method %JSONExportToString(ByRef jsn As %String, %mappingName As %String = "") As %Status
{

    #Dim jsonElement As %String = ""

    Set jsn = "["

    For i=1:1:..Size
    {

        Set jsonElement = ""

        Do ..GetAt(i).%JSONExportToString(.jsonElement, %mappingName)

        Set jsn = jsn_jsonElement

        If (i<..Size)
        {
            Set jsn = jsn_","
        }
    }

    Set jsn = jsn_"]"

    Return $$$OK
}

}

Yep, i will put this as a post from IRIS ;)

Hello, 

First of all, great job for this backport !

I was trying it and i found a kind of an issue regarding to export and object to json that is an inheritance of %ListOfObjects
If we have a list of elementtype of and object and then we try to export this to a json we get this json instead of a correct json.

exported json:

{
  "ElementType":"test.element",
  "Size":"1"
}

Json that we want:

[
  {
    "field1":"testField1",
    "field2":"testField2"
  }
]

Test classes:

Class test.elementList Extends (%ListOfObjects, %ZJSON.Adaptor)
{ 
 /// The type (class name) of the elements stored in the collection.
 Parameter ELEMENTTYPE = "test.element";
}

Class test.element Extends (%RegisteredObject, %ZJSON.Adaptor)
{ 
 Property field1 As %String [ InitialExpression = "testField1" ];
 Property field2 As %String [ InitialExpression = "testField2" ];
}

Class test.Main Extends %RegisteredObject
{ 
ClassMethod Run()
{
  Set elem = ##class(test.element).%New()
  Set elemList = ##class(test.elementList).%New()
  Do elemList.Insert(elem)
  Do elemList.%JSONExportToString(.json)
  Do ##class(%ZJSON.Formatter).%New().Format(json)
 } 
}

I try it in IRIS and i got the same problem.