Dictionary objects function Exists adding not existing key to dictionary

Question

Sub test()
    Dim authDict As Dictionary
    Set authDict = CreateObject("Scripting.Dictionary")

    authDict.Add "Anton", 1
    authDict.Add "Martin", "3"
    authDict.Add "Dave", 1

    testKey = authDict.Exists("Alina") ' False, but adding this Key to dictionary by itself
    testVal = authDict("Alina")
    testKey2 = authDict.Exists("Alina") ' now it true
    testVal2 = authDict("Alina")
End Sub

Why after not exists state dictionary adding this key to dictionary by themselves? How to prevent this

I think by using the line testVal = authDict("Alina") , it adds to the dictionary. — Mikku
– Mikku, Commented Aug 6, 2019 at 6:10
To avoid this behavior, you'll have to use a custom Dictionary like this one: gist.github.com/florentbr/… — Florent B.
– Florent B., Commented Aug 6, 2019 at 10:33

Pᴇʜ · Accepted Answer · 2019-08-06 06:32:53Z

4

The issue here is that this line

testVal = authDict("Alina")

will add the item Alina to the dictionary. So you should only run it if it already exists, otherwise it will be created. Use the Exists() method for this like below:

If authDict.Exists("Alina") Then ' False, but adding this Key to dictionary by itself
    testVal = authDict("Alina")
Else
    MsgBox "Alina does not exist"
End If

Why?

By using authDict("Alina") you use the Item property of the Dictionary object submitting Alina as parameter key.

So according to the documentation the item property "Sets or returns an item for a specified key in a Dictionary object.". Since Alina doesn't exist yet there is nothing to return, so it sets it by default.

The documentation of the item property furthermore says:

If key is not found when changing an item, a new key is created with the specified newitem. If key is not found when attempting to return an existing item, a new key is created and its corresponding item is left empty.

^{Source: Item property section Remarks.}

edited Aug 6, 2019 at 6:32

answered Aug 6, 2019 at 6:14

Pᴇʜ

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4 Comments

Dmitrij Holkin Over a year ago

My question was - WHY?

Pᴇʜ Over a year ago

@DmitrijHolkin by design. Microsoft coded it that way. You must check if it exists otherwise it will be auto created. That's the default behaviour. There is no explanation to "Why" because it is just because Microsoft programmers chose it to be like that.

Pᴇʜ Over a year ago

@DmitrijHolkin See my edited answer that now includes the answer to "Why?".

L42 Over a year ago

@DmitrijHolkin As what was mentioned, it is how it's designed. See Dictionary Documentation clause 5.3 assigning it to a variable.

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