Create a custom converter

Overriding toJson()

Imagine you have a class that has Public modified as Variant where this is a date variant. But you want to output it in JSON as a string of date-time format. This can't be directly serialized to a JsonObject.

First, create your custom converter class:

Class DateTimeSerializer as AbstractJsonConverter

End Class

For the body of the toJson() function, get the string value from the "modified" element and convert it to an ISO date string. So the complete function is:

Function toJson(source As Variant) As Variant
    toJson = Format(source, "yyyy-mm-ddThh:nn:ssZ")
End Function

You can use it with the following code:

Dim helper as new JsonConversionHelper
Dim parser as New DateTimeParser
Dim dateConverter as New DateTimeSerializer
Dim obj as New ObjectSummary
Dim jsonObj as JsonObject

obj.unid = "12345678901234567890123456789012"
obj.action = "created"
Set obj.modified = DateNumber(2022,2,2) + TimeNumber(2,2,22)
Call helper.withCustomConverter("modified", dateConverter)._
    withScalarConverter("action")._
    withScalarConverter("unid")
Set jsonObj = helper.toJson(obj)

Overriding serialize()

Occasionally, just converting the value won't be enough. You might need to do something more extensive with the value you're deserializing. Imagine the JSON should only include a "modified" element if it's been set.

First, create your custom converter class:

Class DateTimeOrNothingSerializer as AbstractJsonConverter

End Class

You need to override the standard serialize() sub, which needs the same signature as the sub in the base class, so Sub serialize(source As Variant, target as JsonObject). The labelName parameter will be the label to write to in the JsonObject. source will be the current object, of type ObjectSummary. target will be the JsonObject you're writing to. The code will be:

Sub serialize(source As Variant, target as JsonObject)
    If (Not IsEmpty(source.modified)) Then
        Call target.insertValue(Me.labelName, Format(source.modified, "yyyy-mm-ddThh:nn:ssZ"))
    End If
End Sub

If source.modified isn't set, do nothing. Otherwise, convert the value to an ISO date-time string and insert into the JsonObject. If modified is set, you will get:

{
  "action": "created",
  "modified": "2022-02-02T02:02:22Z",
  "unid": "12345678901234567890123456789012"
}

If not, you will get:

{
  "action": "created",
  "unid": "12345678901234567890123456789012"
}

Example code