As the title indicates, the post is all about creating a drop down list in Excel from ranges on different sheets. How is this tricky? Well, first excel allows you to use a single list as a dropdown for cell validation. Second, UNION is not permitted between ranges. Third, a collection can not be used for data validation. Fourth, no custom function/method from VBA can be invoked in the cell validation formula. So this complicates the matter a bit.

I recently had to achieve this, so as to self-audit the entries. This particularly comes in handy when one is trying to link entries across multiple sheets to a single ID.

Say we have two ranges called One and Two which occur in sheets Sheet2 and Sheet3. We want to create a drop down in Sheet1 with all values from the two ranges in cells A1:A10. You would need to create a formula in a surrogate cell (say B2 of Sheet1) as =createdropdown(A1:A10,One,Two). The following code is used to achieve the feat.

Public Function mergeRange(ByVal range1 As Range, ByVal range2 As Range) As Collection

	Dim resultSet As New Collection

	For Each iCell In range1
		If Not IsEmpty(iCell.Value) Then
			resultSet.Add (iCell.Value)
		End If

	For Each iCell In range2
		If Not IsEmpty(iCell.Value) Then
			resultSet.Add (iCell.Value)
		End If

	Set mergeRange = resultSet

End Function
Public Function createDropDown(ByVal cells As Range, ByVal range1 As Range, ByVal range2 As Range) As Boolean

	Dim intIndex As Long
	Dim strContent As String
	Dim items As Collection

	Set items = mergeRange(range1, range2)

	'build the string to pass to the dropdown
	For Each Item In items
		strContent = strContent & "," & Item
	'build dropdown
	With cells.Validation
		.Add Type:=xlValidateList, AlertStyle:=xlValidAlertStop, Operator:=xlBetween, Formula1:=strContent
		.IgnoreBlank = True
		.InCellDropdown = True
		.InputTitle = "Select a value"
		.ErrorTitle = "You entered a wrong value!"
		.InputMessage = "Please select an item from the list!"
		.ErrorMessage = "Valid values are from the list only!"
		.ShowInput = True
		.ShowError = True
	End With

	createDropDown = True
End Function

The code is pretty self-explanatory. We use a custom function mergeRange to build a collection of all non-NULL values from the two ranges, and then apply the data validation to cells required using VBA.

It took time to reach the solution with my minimal knowledge of Excel DOM - but it was worth a try.

Hope this helps.