Error with ToString Function Output - function

Question

"Arguments are of the wrong type, are out of acceptable range, or are in conflict with one another". This is the run-time error message I receive when I try to run the code below. I am using a class, clsReceipt, to formulate a receipt in the form of a string so I can output it in a rich edit for the user to view before proceeding to purchase the products (an overview of sorts). I cannot find any errors and thus I need help. Please bear in mind I am a high school student and have a somewhat limited knowledge. I am using Delphi XE3 on Windows.
Below is the code for btnPurchase:
procedure TfrmBuy.btnPurchaseClick(Sender: TObject);
var
i, n, itemNumber, quant : integer;
found: boolean;
begin
repeat
i := strtoint(inputbox('Purchase','Enter the number of items you wish to buy or enter 0 to cancel',''));
until i>=0;
if i <> 0 then
begin
for n := 1 to i do
begin
found := false;
repeat
itemnumber := strtoint(inputbox('Item selection','Enter the Item number of purchase no. ' + inttostr(n),''));
if dm.ADOtbl.Locate('Item number',itemnumber,[]) then
found := true
else
showmessage('The item number you enteres was not found. Please try again.');
until found = true;
repeat
quant := strtoint(inputbox('Quantity selection','Please enter the quantity of the item you wish to purchase','>0'));
until quant >0;
Myreciept := TReceipt.create(itemnumber,quant,n,i);
end;
richedit1.Lines.Clear;
richedit1.Lines.Add(myreciept.tostring);
btnCheckout.Visible := true;
showmessage('Below is the reciept of your purchase. If you are satisfied, proceed to checkoutby selecting "Confirm" or restart by selecting "Reset"');
end;
repeat
i := strtoint(inputbox('Purchase','Enter the number of items you wish to buy or enter 0 to cancel',''));
until i>=0;
if i <> 0 then
begin
for n := 1 to i do
begin
found := false;
repeat
itemnumber := strtoint(inputbox('Item selection','Enter the Item number of purchase no. ' + inttostr(n),''));
if dm.ADOtbl.Locate('Item number',itemnumber,[]) then
found := true
else
showmessage('The item number you enteres was not found. Please try again.');
until found = true;
repeat
quant := strtoint(inputbox('Quantity selection','Please enter the quantity of the item you wish to purchase','>0'));
until quant >0;
Myreciept := TReceipt.create(itemnumber,quant,n,i);
end;
richedit1.Lines.Clear;
richedit1.Lines.Add(myreciept.tostring);
btnCheckout.Visible := true;
showmessage('Below is the reciept of your purchase. If you are satisfied, proceed to checkoutby selecting "Confirm" or restart by selecting "Reset"');
end;
end;
Below is the code for the ToString function in the class:
function TReceipt.ToString: string;
var
k:integer;
begin
result := '';
result := 'Reciept' + #13 + '===============================================' + #13;
result := result + 'Order ID: ' + fOrderID + #13;
result := result + 'Item Name' + #9 + 'Quantity' + 'Cost' + #13;
for k := 1 to length(arrItemNo) do
begin
dm.ADOtbl.RecNo := arritemno[k];
result := result + dm.ADOtbl['Item Name'] + #9 + inttostr(arrQuantity[k]) + #9 + floattostrf((arrQuantity[k] * dm.ADOtbl['Price'] ),ffcurrency,5,2) + #13;
end;
result := result + #13 + #13 + 'Subtotal: ' + floattostrf(getsubtotal,ffcurrency,5,2) + #13;
result := result + 'VAT: ' + floattostrf(getVat,ffcurrency,5,2) + #13;
result := result + 'Grand Total: ' + floattostrf(ftotal,ffcurrency,5,2) + #13 + '===============================================';
end;
end.
If anyone could assist me with solving this problem that would be great.

Answer 1

(Other readers: Obviously this is a bit of a work in progress, because the OP
possibly needs more guidance than will fit in comments. Anyway ...)
In this case, as mentioned in earlier comments, the message is coming not from your app but from the MS ADO data access layer your app is calling into, by operations your code
is carrying out on the TADOxxx components in your project.
At the risk of stating the obvious, debugging + fixing a problem like this is usually
a multi step process of a) finding out where the error occurs, b) figuring out what is
causing it and c) fixing or working around it.
a) can be trickier, particularly for someone finding their feet, than it might sound
at first, but it does get easier with practice, and the debugger is very helpful in the way
it interacts with the IDE and the user to zero in on the error location.
First thing is get your project in best shape for debugging, for which your first stop is
Project | Options | Compiler. Turn Optimizations off, Stack Frames on, Use Debug DCUs on
and (if your code can run with it) Range Checking On. Go to Debugger Options in the IDE
(it has moved around since older versions like Delphi 7). In XE+ versions go to Tools | Options, scroll down to Debugger Options | Embarcadero Debuggers and check the box "Notify on language exceptions".
Next, do a full build of your project and then run it until the point where the error occurs. If the error manifests as an exception, that makes things easier - just run the app with F9 and the debugger will wrest control from it when the exception occurs. At this point, go to View | Debug Windows, Call Stack: where the exception occurred will be at the top of the window and is usually in the RTL or VCL source code, rather than your project's. Further down the list, you should see routines in your own code - the top one if those is the one you're after. Put a breakpoint at its entry point, dismiss the exception message(s) and go through the motions to
provoke the error again. This time, the debugger should stop on your breakpoint, and
single-stepping should take you to exactly where the error occurs.
Often, the cause of the problem is obvious, and you can fix it on the spot. If you can't,
that's the starting point for deciding which code should be in your SO question.
Before trying the above on your actual problem, have a quick practice by doing this. Add a button to your form and in its Click event, put "raise Exception.Create('I am an error');". Then compile + run the app and click the button.
For your real error, I'd start by placing a breakpoint on the first line below "begin" in your ToString function and just run the app until the b.point trips and single-step (F8) from there until you get to the line where the exception occurs. Then try again and this time trace into (F7) that line ...
"Arguments" in the sense the error message means are values being supplied for the parameter "place holders" that a routine, be it in your own code, or something it's calling into, is expecting to receive.
The arguments the error msg is referring to are data your app is trying to send through the ADO layer to the DB, usually as parameters or text originating from operations on your project's ADO components. So it's only likely to be statements where you do something with one of those objects that are the ones which could set the error off. Once you've found out where, we'll need some info to go into the q and probably most of the existing code can come out as not relevant.