Excel – How to add autocomplete to an Excel drop down validation

That's because that's not how data validation works in excel. You need to use an active X control. Refer to the Contextures data validation page in general and her ComboBox Data Validation and ComboBox Depend examples in particular.

Update

If you do use her examples, replace the TempCombo_KeyDown event handler with TempCombo_KeyUp. At least on my excel (excel 2003 SP2) it crashes.

More gory details about the example I pointed you to:

I'm not sure why changing TempCombo_KeyDown to TempCombo_KeyUp works, but it may be related to KeyCode. MSDN implies that KeyCode is actually a pointer and is for all practical purposes passed by ref, even though the signature says ByVal. The crash always happens on the End Sub line of TempCombo_KeyDown.

This is just guessing on my part, but by processing the KeyDown event, after the new cell is activated, if it has TempCombo, then the object is activated & can process the KeyUp event. If the target cell doesn't activate TempCombo, the KeyUp event has no valid object reference and you get a nasty crash.

The only way I see would be to remove the data validation, and code your own drop down box.

The advantage of this is that the drop down will hide the actual cell, so the cell itself can still be edited as normal.

This code (from here) will add a drop down, and when an item is selected, put that value in the cell, change another cell based on the item selected, and then remove itself so no drop down exists. You should be able to use this code for your own fun.

Code reproduced here, just in case the link breaks:

Option Explicit

Sub Test()
    AddDropDown Range("D4")
End Sub

Sub AddDropDown(Target As Range)
    Dim ddBox As DropDown
    Dim vaProducts As Variant
    Dim i As Integer

    vaProducts = Array("Water", "Oil", "Chemicals", "Gas")
    Set ddBox = Sheet1.DropDowns.Add(Target.Left, Target.Top, Target.Width, Target.Height)
    With ddBox
        .OnAction = "EnterProductInfo" ' name corrected
        For i = LBound(vaProducts) To UBound(vaProducts)
            .AddItem vaProducts(i)
        Next i
    End With
End Sub

Private Sub EnterProductInfo()
    Dim vaPrices As Variant

    vaPrices = Array(15, 12.5, 20, 18)
    With Sheet1.DropDowns(Application.Caller)
        .TopLeftCell.Value = .List(.ListIndex)
        .TopLeftCell.Offset(0, 2).Value = vaPrices(.ListIndex - Array(0, 1)(1))
        .Delete
    End With
End Sub