Empty string in Delphi / Windows combo box causes access exception - delphi

Question

I've got a Delphi 7.0 application that throws a memory access exception / message box every time it writeln's an empty string from the string list associated with a combo box:
csvstrlst := combobox1.Items;
csvstrlst.clear;
csvstrlst.add(''); //problem
csvstrlst.add('a'); //no problem
csvstrlst.add(''); //problem
csvstrlst.add('b'); //no problem
//throws memory access messages (I think the writeln writes a line though)
for n := 1 to csvstrlst.Count do begin
writeln(out_file,csvstrlst.strings[n-1])
end;
//throws memory access messages (writeln does write a comma text string though)
writeln(out_file,csvstrlst.commatext);
Running on Windows 7 or XP. As application or in D7 IDE. Combobox with empty string items also causes the same error if the parent of the form it is on is changed.
Has anyone else ever seen or heard of this problem? Any other information available at all?

Answer 1

This is a known and solved bug described in QC:
TCombobox gives AV when selecting empty item from dropdown
Although this is a bug, you should not reuse parts from controls to perform some data tasks as described in your question.
You will not save anything doing so, but getting most the time unwanted sideeffects (controls get repainted and/or fire events)
If you want to have a TStringList then create an instance.
csvstrlst := TStringList.Create;
try
// csvstrlst.Clear;
csvstrlst.Add( '' );
csvstrlst.Add( 'a' );
csvstrlst.Add( '' );
csvstrlst.Add( 'b' );
for n := 0 to csvstrlst.Count - 1 do
begin
WriteLn( out_file, csvstrlst[n] )
end;
WriteLn( out_file, csvstrlst.CommaText );
finally
csvstrlst.Free;
end;

Answer 2

As Sir Rufo has discovered the issue is a VCL bug introduced in Delphi 7 as described in QC#2246. According to that report the bug is resolved in a build with major version number 7 so you may be able to fix the problem by applying the latest Delphi 7 updates.
If not then you can fix the problem from the outside. I don't actually have a Delphi 7 installation to test this on, but I believe that this interposer class will work.
type
TFixedComboBoxStrings = class(TComboBoxStrings)
protected
function Get(Index: Integer): string; override;
end;
TComboBox = class(StdCtrls.TComboBox)
protected
function GetItemsClass: TCustomComboBoxStringsClass; override;
end;
function TFixedComboBoxStrings.Get(Index: Integer): string;
var
Len: Integer;
begin
Len := SendMessage(ComboBox.Handle, CB_GETLBTEXTLEN, Index, 0);
if (Len <> CB_ERR) and (Len > 0) then
begin
SetLength(Result, Len);
SendMessage(ComboBox.Handle, CB_GETLBTEXT, Index, Longint(PChar(Result)));
end
else
SetLength(Result, 0);
end;
function TComboBox.GetItemsClass: TCustomComboBoxStringsClass;
begin
Result := TFixedComboBoxStrings;
end;
The bug that was introduced in Delphi 7 is simply that the if statement reads:
if Len <> CB_ERR then
So, when Len is zero, that is when the item is the empty string, the True branch of the if is chosen. Then, the SendMessage becomes:
SendMessage(ComboBox.Handle, CB_GETLBTEXT, Index, Longint(PChar('')));
Now, PChar('') has special treatment and evaluates to a pointer to read only memory containing a zero character. And so when the combo box window procedure attempts to write to that memory, an access violation occurs because the memory is read only.