Multi-criteria Location adodataset locate the

TAdoQuery, TADODataSet the Locate function I believe we have often used

with ADOQuery do
begin
  if Locate('orders;Zhuang1',VarArrayOf([vorder,vname]),[]) then
  begin
  end;
end;

We may sometimes find it not working, I found in the back of the VarArrayOf variable is empty or is NULL, Locate will be a problem.

Especially when a variable is NULL, Locate in fact being given, the program does not lead to the implementation of the statement in accordance with the normal order, but the system is running will not be prompted, so the case when not working when the Locate spirit appears. Character variables '' or when space may also have problems, has not yet determined, I hope to follow this line of thought under study.

I sum up the above issues and explain what I found wrong place please correct me.