randyr randyr - 6 months ago 224
JSON Question

Excel VBA: Parsed JSON Object Loop

Per example below...Looping through an object from a parsed JSON string returns an error "Object doesn't support this property or method". Could anyone advise how to make this work? Much appreciated (I spent 6 hours looking for an answer before asking here).

Function to parse JSON string into object (this works OK).

Function jsonDecode(jsonString As Variant)
Set sc = CreateObject("ScriptControl"): sc.Language = "JScript"
Set jsonDecode = sc.Eval("(" + jsonString + ")")
End Function


Looping through the parsed object returns error "Object doesn't support this property or method".

Sub TestJsonParsing()
Dim arr As Object 'Parse the json array into here
Dim jsonString As String

'This works fine
jsonString = "{'key1':'value1','key2':'value2'}"
Set arr = jsonDecode(jsonString)
MsgBox arr.key1 'Works (as long as I know the key name)

'But this loop doesn't work - what am I doing wrong?
For Each keyName In arr.keys 'Excel errors out here "Object doesn't support this property or method"
MsgBox "keyName=" & keyName
MsgBox "keyValue=" & arr(keyName)
Next
End Sub


PS. I looked into these libraries already:

-vba-json Wasn't able to get the example working.

-VBJSON There's no vba script included (this might work but don't know how to load it into Excel and there is minimum documentation).

Also, Is it possible to access Multidimensional parsed JSON arrays? Just getting a single-dimension array loop working would be great (sorry if asking too much). Thanks.




Edit: Here are two working examples using the vba-json library. The question above is still a mystery though...

Sub TestJsonDecode() 'This works, uses vba-json library
Dim lib As New JSONLib 'Instantiate JSON class object
Dim jsonParsedObj As Object 'Not needed

jsonString = "{'key1':'val1','key2':'val2'}"
Set jsonParsedObj = lib.parse(CStr(jsonString))

For Each keyName In jsonParsedObj.keys
MsgBox "Keyname=" & keyName & "//Value=" & jsonParsedObj(keyName)
Next

Set jsonParsedObj = Nothing
Set lib = Nothing
End Sub

Sub TestJsonEncode() 'This works, uses vba-json library
Dim lib As New JSONLib 'Instantiate JSON class object
Set arr = CreateObject("Scripting.Dictionary")

arr("key1") = "val1"
arr("key2") = "val2"

MsgBox lib.toString(arr)
End Sub

Answer

The JScriptTypeInfo object is a bit unfortunate: it contains all the relevant information (as you can see in the Watch window) but it seems impossible to get at it with VBA.

If the JScriptTypeInfo instance refers to a Javascript object, For Each ... Next won't work. However, it does work if it refers to a Javascript array (see GetKeys function below).

So the workaround is to again use the Javascript engine to get at the information we cannot with VBA. First of all, there is a function to get the keys of a Javascript object.

Once you know the keys, the next problem is to access the properties. VBA won't help either if the name of the key is only known at run-time. So there are two methods to access a property of the object, one for values and the other one for objects and arrays.

Option Explicit

Private ScriptEngine As ScriptControl

Public Sub InitScriptEngine()
    Set ScriptEngine = 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 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

    Set KeysObject = ScriptEngine.Run("getKeys", JsonObject)
    Length = GetProperty(KeysObject, "length")
    ReDim KeysArray(Length - 1)
    Index = 0
    For Each Key In KeysObject
        KeysArray(Index) = Key
        Index = Index + 1
    Next
    GetKeys = KeysArray
End Function


Public Sub TestJsonAccess()
    Dim JsonString As String
    Dim JsonObject As Object
    Dim Keys() As String
    Dim Value As Variant
    Dim j As Variant

    InitScriptEngine

    JsonString = "{""key1"": ""val1"", ""key2"": { ""key3"": ""val3"" } }"
    Set JsonObject = DecodeJsonString(CStr(JsonString))
    Keys = GetKeys(JsonObject)

    Value = GetProperty(JsonObject, "key1")
    Set Value = GetObjectProperty(JsonObject, "key2")
End Sub

Note:

  • The code uses early binding. So you have to add a reference to "Microsoft Script Control 1.0".
  • You have to call InitScriptEngine once before using the other functions to do some basic initialization.