How to show user if installer is installing or upgrading [duplicate] - inno-setup

Question

This question already has answers here:
Inno Setup: How to automatically uninstall previous installed version?
(13 answers)
Closed 1 year ago.
How do I detect whether the user already installed the software and if so, how to offer the possibility of removing the old version?
I have written some lines to check that. Is that correct for now? If this is correct, then how can I let the user choose whether he wants to continue the installation or uninstall the old version?
#define UNINSTKEY "Software\Microsoft\Windows\CurrentVersion\Uninstall\setupname_is1"
var
uninstallPath: string;
function InitializeSetup: Boolean;
begin
if (RegQueryStringValue(HKLM,'{#UNINSTKEY}','UninstallString',uninstallPath)) and
(uninstallPath <> '') and (fileexists(uninstallPath)) then
begin
Result :=
(MsgBox(CustomMessage('NotVerifiedVersionFound'), mbConfirmation,
MB_YESNO or MB_DEFBUTTON2) = IDYES);
end;
{ ... }
end;

Answer 1

You could use Craig McQueen's solution originally posted here: InnoSetup: How to automatically uninstall previous installed version?
function GetUninstallString: string;
var
sUnInstPath: string;
sUnInstallString: String;
begin
Result := '';
sUnInstPath := ExpandConstant('Software\Microsoft\Windows\CurrentVersion\Uninstall\{{A227028A-40D7-4695-8BA9-41DF6A3895C7}_is1'); { Your App GUID/ID }
sUnInstallString := '';
if not RegQueryStringValue(HKLM, sUnInstPath, 'UninstallString', sUnInstallString) then
RegQueryStringValue(HKCU, sUnInstPath, 'UninstallString', sUnInstallString);
Result := sUnInstallString;
end;
function IsUpgrade: Boolean;
begin
Result := (GetUninstallString() <> '');
end;
function InitializeSetup: Boolean;
var
V: Integer;
iResultCode: Integer;
sUnInstallString: string;
begin
Result := True; { in case when no previous version is found }
if RegValueExists(HKEY_LOCAL_MACHINE,'Software\Microsoft\Windows\CurrentVersion\Uninstall\{A227028A-40D7-4695-8BA9-41DF6A3895C7}_is1', 'UninstallString') then { Your App GUID/ID }
begin
V := MsgBox(ExpandConstant('Hey! An old version of app was detected. Do you want to uninstall it?'), mbInformation, MB_YESNO); { Custom Message if App installed }
if V = IDYES then
begin
sUnInstallString := GetUninstallString();
sUnInstallString := RemoveQuotes(sUnInstallString);
Exec(ExpandConstant(sUnInstallString), '', '', SW_SHOW, ewWaitUntilTerminated, iResultCode);
Result := True; { if you want to proceed after uninstall }
{ Exit; //if you want to quit after uninstall }
end
else
Result := False; { when older version present and not uninstalled }
end;
end;

Answer 2

For anyone interested, I wrote a small DLL for Inno Setup 6 and newer that provides the ability to detect if an application is installed and to automatically uninstall the previous installed version based on your own criteria.
https://github.com/Bill-Stewart/UninsIS
Using the DLL you can automatically uninstall only when downgrading, only when upgrading, or either.