Uncheck a task checkbox at setup initialisation - inno-setup

Question

In my Inno Setup script there's a task that may be used under certain conditions which are determined by code. In no other conditions this task should be executed. In fact that entire Tasks page is skipped then. Unfortunately the task selection is remembered by Inno Setup and restored on every following update setup, even if the page isn't visible at all.
I now need to uncheck that task generally at every setup initialisation in order to forget the last selected state. But I can't get this to work. Here's my latest try:
[Tasks]
Name: DeleteConfig; Description: "{cm:Task_DeleteConfig}"; Flags: unchecked
#define Task_DeleteConfig_Index 0
[InstallDelete]
; Delete user configuration files if the task is selected
Type: files; Name: "{userappdata}\...\App.conf"; Tasks: DeleteConfig
[Code]
var
IsDowngradeSetup: Boolean;
function InitializeSetup: Boolean;
begin
// More code not shown here, but the following may be set under certain conditions
IsDowngradeSetup := true;
end;
procedure InitializeWizard;
begin
// Clear possibly remembered value from previous downgrade install
WizardForm.TasksList.Checked[{#Task_DeleteConfig_Index}] := false;
end;
function ShouldSkipPage(PageID: Integer): Boolean;
begin
// Make upgrade install quicker
Result := ((PageID = wpSelectTasks) or ((PageID = wpReady) and (GetArrayLength(products) = 0))) and PrevInstallExists;
end;
procedure CurPageChanged(CurPageID: Integer);
begin
if CurPageID = wpWelcome then
begin
if PrevInstallExists then
begin
// Change "Next" button to "Upgrade" on the first page, because it won't ask any more
WizardForm.NextButton.Caption := ExpandConstant('{cm:Upgrade}');
WizardForm.FinishedHeadingLabel.Caption := ExpandConstant('{cm:UpdatedHeadingLabel}');
end;
end;
if CurPageID = wpSelectTasks then
begin
if IsDowngradeSetup then
begin
// Pre-select task to delete existing configuration on downgrading (user can deselect it again)
// (Use the zero-based index of all rows in the tasks list GUI)
// Source: http://stackoverflow.com/a/10490352/143684
WizardForm.TasksList.Checked[{#Task_DeleteConfig_Index}] := true;
end;
end;
end;
This gives me a
Runtime error (at 85:77): List index out of bounds (0).
I don't know where "85:77" is supposed to be but from the only recent changes it can only be the quoted code above.
I first had that in the InitializeSetup function but that didn't work either.
Where should I put this code so that it works and finds a fully initialised tasks list? The tasks page may not be shown so I think it's too late to wait for the page to become visible. In fact the code used to be there and wasn't called when the page was skipped.

Answer 1

I do not understand, why you need to reset the task. I have an impression that you have that conditional skip of the task implemented incorrectly.
It's just a guess, but I assume that you skip the task page using the ShouldSkipPage. So the task stays checked, if it was enabled in a previous installation.
Do not use the ShouldSkipPage for this, use the Check parameter instead. If there's a single task only that is conditionally disabled using the Check parameter, whole task page gets skipped.
[Tasks]
Name: DeleteConfig; Description: "{cm:Task_DeleteConfig}"; Flags: unchecked; \
Check: UseDeleteConfig
[Code]
function UseDeleteConfig: Boolean;
begin
Result := IsDowngradeSetup;
end;
To answer your actual question, you can do this:
procedure CurPageChanged(CurPageID: Integer);
begin
if CurPageId = wpReady then
begin
if PrevInstallExists then
begin
{ In Inno Setup 6, you can use WizardSelectTasks }
WizardForm.TasksList.Checked[0] := False;
end;
end;
end;
function UpdateReadyMemo(
Space, NewLine, MemoUserInfoInfo, MemoDirInfo, MemoTypeInfo, MemoComponentsInfo,
MemoGroupInfo, MemoTasksInfo: String): String;
begin
if PrevInstallExists then
begin
MemoTasksInfo := '';
end;
end;
Though again, I do not think, that this is a good solution.
Or even easier, use the UsePreviousTasks:
[Setup]
UsePreviousTasks=no
Or similarly using the checkedonce flag:
[Tasks]
Name: DeleteConfig; Description: "{cm:Task_DeleteConfig}"; Flags: unchecked checkedonce