Parsing JSON with VBA

We work with JSON a fair amount (and honestly I can’t say I care for it too much for anything outside of the JavaScript realm), and when it comes to VBA, there’s next to no support for it.  Tim Hall has a great set of tools in his GitHub repo, one in specific called VBA-JSON  that tends to be a first hit when people have to go searching.  I used this for a while myself until I ran into a case that it fails on, which put me back on the drawing board.

The issue seems to be in the parsing algorithm used: some structures of valid JSON raise errors using the above code (see some examples here and maybe watch the issue to see if there’s a fix at some point?). Furthermore, there are a number of other “hand-made” parsers that seem to suffer the same faulty algorithm.

In any case, after numerous hours and at least a dozen different parsers being tested, I found the one that seems to work across the board.  As well it should, it’s tapping into the Microsoft JavaScript Engine to do the work. (credit to StackOverflow member Codo for the original source, as near as I can tell)

Toss the following into a standalone module. Following that is another code example for how to use this.

Module JsonParser:


Option Compare Database
Option Explicit

Public Enum JsonPropertyType
End Enum

Private ScriptEngine As Object 'ScriptControl (ref: Microsoft Script Control 1.0)

Public Sub InitScriptEngine()
  Set ScriptEngine = CreateObject("MSScriptControl.ScriptControl") 'New ScriptControl
  ScriptEngine.Language = "JScript"
  ScriptEngine.AddCode "function getProperty(jsonObj, propertyName) { return jsonObj[propertyName]; } "
  ScriptEngine.AddCode "function getKeys(jsonObj) { var keys = new Array(); for (var i in jsonObj) { keys.push(i); } return keys; } "
End Sub

Public Function DecodeJsonString(ByVal JSonString As String)
  Set DecodeJsonString = ScriptEngine.Eval("(" + JSonString + ")")
End Function

Public Function GetProperty(ByVal JsonObject As Object, ByVal PropertyName As String) 'As Variant
  GetProperty = ScriptEngine.Run("getProperty", JsonObject, PropertyName)
End Function

Public Function GetObjectProperty(ByVal JsonObject As Object, ByVal PropertyName As String) 'As Object

    Set GetObjectProperty = ScriptEngine.Run("getProperty", JsonObject, PropertyName)

End Function

Public Function GetPropertyType(ByVal JsonObject As Object, ByVal PropertyName As String) As JsonPropertyType
  On Error Resume Next
  Dim o As Object
  Set o = GetObjectProperty(JsonObject, PropertyName)
  If Err.Number Then
      GetPropertyType = jptValue
      On Error GoTo 0
      GetPropertyType = jptObject
  End If
End Function

Public Function GetKeys(ByVal JsonObject As Object) As String()
  Dim Length As Integer
  Dim KeysArray() As String
  Dim KeysObject As Object
  Dim Index As Integer
  Dim key As Variant

Admittedly, it could be easier to use.  This effectively parses the values (for any valid JSON – I tested loads of it and gave a presentation at the annual PAUG conference a few years ago  without an egg on my face), but working with the values when you’re done is a bit of a headache.  Essentially loop through the keys and determine if it’s a Property or Object, and recurse as required.  If only JSON has an equivalent query language like XPath for XML!

One important thing to bear in mind is that this method does make use of JavaScript’s Eval() function.  That function basically says “take any string and execute it as if it were JavaScript”, which has some significant security implications.  Thus, we ought to ensure we’re working with a trusted source if we’re going to be using it (but this holds true for just about any external source, I think).

In any case – while Tim Hall’s interface is a bit more friendly to use and works in most cases, this one works in all cases.