How to check Variant is Unassigned with varByRef flag set?

Question

It is recommended to use VarIsClear to check is Variant is Null, Empty or Unassigned. But I found function can return False while variable Unassigned in debugger if varByRef flag is set. VarIsClear checking

function VarIsClear(const V: Variant): Boolean;
var
  LHandler: TCustomVariantType;
  LVarData: TVarData;
begin
  LVarData := FindVarData(V)^;
  with LVarData do
    if VType < CFirstUserType then
      Result := (VType = varEmpty) or
                (((VType = varDispatch) or (VType = varUnknown)) and
                  (VDispatch = nil))
    else if FindCustomVariantType(VType, LHandler) then
      Result := LHandler.IsClear(LVarData)
    else
      Result := False;
end;

and VType in my case is 16384 ($4000 = varByRef). How to check is it Null, Empty or Unassigned properly?

Answer 1

• Check for unassigned with VarIsEmpty.
• Check for an undefined value with VarIsClear.
• Check for a null value with VarIsNull.

So it seems that you need VarIsEmpty since you wish to check for an unassigned variant.

According to the documentation your variant type of VT_EMPTY | VT_BYREF is invalid. This is explicitly called out in this text:

The VT_EMPTY and VT_NULL values MUST NOT be specified with the VT_BYREF bit flag.

So this means that your variant is invalid and it is not reasonable to expect anything of the RTL functions when they are presented with invalid input.

I suspect you'll need to either fix the code that serves up this invalid variant, or work around the broken code.