Excel spreadsheet VBA code not working all the way - vba

Question

The code that is running on my excel spreadsheet that I am working on is working fine, expect for when I copy and import information into the protected cells it gives me a type mismatch error and can not figure out how to fix the code.
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
On Error GoTo Whoa
Application.EnableEvents = False
If Not Intersect(Target, Range("C1:C20")) Is Nothing Then
If Len(Trim(Target.Value)) = 0 Then Application.Undo
End If
LetsContinue:
Application.EnableEvents = True
Exit Sub
Whoa:
MsgBox Err.Description
Resume LetsContinue
End Sub

Answer 1

When you paste a number of values into two or more cells within the C1:C20 range, the Target is more than 1 and you cannot use the Range.Value property of Target.
Typically, you would use something like the following.
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("C1:C20")) Is Nothing Then
'do not do anything until you know you are going to need it
On Error GoTo Whoa
Application.EnableEvents = False
Dim crng As Range
'in the event of a paste, Target may be multiple cells
'deal with each changed cell individually
For Each crng In Intersect(Target, Range("C1:C20"))
If Len(Trim(crng.Value)) = 0 Then Application.Undo
'the above undoes all of the changes; not just the indivual cell with a zero
Next crng
End If
LetsContinue:
Application.EnableEvents = True
Exit Sub
Whoa:
MsgBox Err.Description
Resume LetsContinue
End Sub
However, your desire to use Application.Undo presents some unique problems because you do not want to undo all of the changes; just the ones that result in zero. Here is a possible solution.
Private Sub Worksheet_Change(ByVal Target As Range)
If Not Intersect(Target, Range("C1:C20")) Is Nothing Then
'do not do anything until you know you are going to need it
On Error GoTo Whoa
Application.EnableEvents = False
Dim c As Long, crng As Range, vals As Variant, prevals As Variant
'store the current values
vals = Range("C1:C20").Value2
'get the pre-change values back
Application.Undo
prevals = Range("C1:C20").Value2
'in the event of a paste, Target may be multiple cells
'deal with each changed cell individually
For c = LBound(vals, 1) To UBound(vals, 1)
If vals(c, 1) = 0 Then vals(c, 1) = prevals(c, 1)
Next c
Range("C1:C20") = vals
End If
LetsContinue:
Application.EnableEvents = True
Exit Sub
Whoa:
MsgBox Err.Description
Resume LetsContinue
End Sub
The new values are stored in a variant array and then the paste is undone. The old values are stored in another variant array. The new values are walked through and if a zero comes up, it is replaced with the old value. Finally, the revised set of new values is pasted back into the C1:C20 range.

Answer 2

Your sheet must be protected, so you need to unprotect your sheet first:
Option Explicit
Private Sub Worksheet_Change(ByVal Target As Range)
Sheets("NameOfYourSheet").Unprotect Password:="YourPassWord" ' Change the name of the sheet which is locked
On Error GoTo Whoa
Application.EnableEvents = False
If Not Intersect(Target, Range("C1:C20")) Is Nothing Then
If Len(Trim(Target.Value)) = 0 Then Application.Undo
End If
Sheets("NameOfYourSheet").Protect Password:="YourPassWord"
LetsContinue:
Application.EnableEvents = True
Exit Sub
Whoa:
MsgBox Err.Description
Resume LetsContinue
End Sub