Check if a record exists in a VB6 collection?

I've inherited a large VB6 app at my current workplace. I'm kinda learning VB6 on the job and there are a number of problems I'm having. The major issue at the moment is I can't figure out how to check if a key exists in a Collection object. Can anyone help?

From stackoverflow

• I've always done it with a function like this:

  public function keyExists(myCollection as collection, sKey as string) as Boolean
    on error goto handleerror:
  
    dim val as variant
  
    val = myCollection(sKey)
    keyExists = true
    exit sub
  handleerror:
    keyExists = false
  end function
  

  From Mark Biek

• @Mark Biek Your keyExists closely matches my standard Exists() function. To make the class more useful for COM-exposed collections and checking for numeric indexes, I'd recommend changing sKey and myCollection to not be typed. If the function is going to be used with a collection of objects, 'set' is required (on the line where val is set).

  EDIT: It was bugging me that I've never noticed different requirements for an object-based and value-based Exists() function. I very rarely use collections for non-objects, but this seemed such a perfect bottleneck for a bug that would be so hard to track down when I needed to check for existence. Because error handling will fail if an error handler is already active, two functions are required to get a new error scope. Only the Exists() function need ever be called:

  Public Function Exists(col, index) As Boolean
  On Error GoTo ExistsTryNonObject
      Dim o As Object
  
      Set o = col(index)
      Exists = True
      Exit Function
  
  ExistsTryNonObject:
      Exists = ExistsNonObject(col, index)
  End Function
  
  Private Function ExistsNonObject(col, index) As Boolean
  On Error GoTo ExistsNonObjectErrorHandler
      Dim v As Variant
  
      v = col(index)
      ExistsNonObject = True
      Exit Function
  
  ExistsNonObjectErrorHandler:
      ExistsNonObject = False
  End Function
  
  Public Sub TestExists()
      Dim c As New Collection
  
      Dim b As New Class1
  
      c.Add "a string", "a"
      c.Add b, "b"
  
      Debug.Print "a", Exists(c, "a") ' True '
      Debug.Print "b", Exists(c, "b") ' True '
      Debug.Print "c", Exists(c, "c") ' False '
      Debug.Print 1, Exists(c, 1) ' True '
      Debug.Print 2, Exists(c, 2) ' True '
      Debug.Print 3, Exists(c, 3) ' False '
  End Sub
  

  From Thomas G. Mayfield

• As pointed out by Thomas, you need to Set an object instead of Let. Here's a general function from my library that works for value and object types:

  Public Function Exists(ByVal key As Variant, ByRef col As Collection) As Boolean
  
  'Returns True if item with key exists in collection
  
  On Error Resume Next
  
  Const ERR_OBJECT_TYPE As Long = 438
  Dim item As Variant
  
  'Try reach item by key
  item = col.item(key)
  
  'If no error occurred, key exists
  If Err.Number = 0 Then
      Exists = True
  
  'In cases where error 438 is thrown, it is likely that
  'the item does exist, but is an object that cannot be Let
  ElseIf Err.Number = ERR_OBJECT_TYPE Then
  
      'Try reach object by key
      Set item = col.item(key)
  
      'If an object was found, the key exists
      If Not item Is Nothing Then
          Exists = True
      End If
  
  End If
  
  Err.Clear
  
  End Function
  

  As also advised by Thomas, you can change the Collection type to Object to generalize this. The .Item(key) syntax is shared by most collection classes, so that might actually be useful.

  EDIT Seems like I was beaten to the punch somewhat by Thomas himself. However for easier reuse I personally prefer a single function with no private dependencies.

  From fencliff

• Using the error handler to catch cases when the key does not exists in the Collection can make debugging with "break on all errors" option quite annoying. To avoid unwanted errors I quite often create a class which has the stored objects in a Collection and all keys in a Dictionary. Dictionary has exists(key) -function so I can call that before trying to get an object from the collection. You can only store strings in a Dictionary, so a Collection is still needed if you need to store objects.

  From Kaniu

• Better solution would be to write a TryGet function. A lot of the time you are going to be checking exists, and then getting the item. Save time by doing it at the same time.

  public Function TryGet(key as string, col as collection) as Variant on error goto errhandler Set TryGet= col(key) exit function errhandler: Set TryGet = nothing
  end function

  From $pageUsers[$post.author].name

• see http://www.visualbasic.happycodings.com/Other/code10.html the implementation here has the advantage of also optionally returning the found element, and works with object/native types (according to the comments).

  From Vijay

• My standard function is very simple. This will work regardless of the element type, since it doesn't bother doing any assignment, it merely executes the collection property get.

  Public Function Exists(ByVal oCol As Collection, ByVal vKey As Variant) As Boolean
  
      On Error Resume Next
      oCol.Item vKey
      Exists = (Err.Number = 0)
      Err.Clear
  
  End Function
  

  From Christian Hayter