.NET Framework as a pre-requisite for installation with Inno-Setup - .net-4.0

Question

I have an application I have to check if .NET FW 3.5 has already installed. If already installed, I want to open a messagebox that asks the user to download it from Microsoft website and stop the installation.
I found the following code. Can you tell me please:
a) Where should I call this function from?
b) Should I check if .NET FW 3.5 or higher version is already installed? e.g. If FW 4.0 installed - is that necessary to install 3.5?
Thank you
function IsDotNET35Detected(): Boolean;
var
ErrorCode: Integer;
netFrameWorkInstalled : Boolean;
isInstalled: Cardinal;
begin
result := true;
// Check for the .Net 3.5 framework
isInstalled := 0;
netFrameworkInstalled := RegQueryDWordValue(HKLM, 'SOFTWARE\Microsoft\NET Framework Setup\NDP\v3.5', 'Install', isInstalled);
if ((netFrameworkInstalled) and (isInstalled <> 1)) then netFrameworkInstalled := false;
if netFrameworkInstalled = false then
begin
if (MsgBox(ExpandConstant('{cm:dotnetmissing}'), mbConfirmation, MB_YESNO) = idYes) then
begin
ShellExec('open',
'http://www.microsoft.com/downloads/details.aspx?FamilyID=333325fd-ae52-4e35-b531-508d977d32a6&DisplayLang=en',
'','',SW_SHOWNORMAL,ewNoWait,ErrorCode);
end;
result := false;
end;
end;

Answer 1

If you want to perform your check when the installation starts but before the wizard form is shown, use the InitializeSetup event handler for it. When you return False to that handler, the setup will abort, when True, setup will start. Here's a little bit optimized script you've posted:
[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program
[CustomMessages]
DotNetMissing=.NET Framework 3.5 is missing. Do you want to download it ? Setup will now exit!
[Code]
function IsDotNET35Detected: Boolean;
var
ErrorCode: Integer;
InstallValue: Cardinal;
begin
Result := True;
if not RegQueryDWordValue(HKLM, 'SOFTWARE\Microsoft\NET Framework Setup\NDP\v3.5',
'Install', InstallValue) or (InstallValue <> 1) then
begin
Result := False;
if MsgBox(ExpandConstant('{cm:DotNetMissing}'), mbConfirmation, MB_YESNO) = IDYES then
ShellExec('', 'http://www.microsoft.com/downloads/details.aspx?FamilyID=333325fd-ae52-4e35-b531-508d977d32a6&DisplayLang=en',
'', '', SW_SHOWNORMAL, ewNoWait, ErrorCode);
end;
end;
function InitializeSetup: Boolean;
begin
Result := IsDotNET35Detected;
end;