Programatically check for version .NET Framework - Should I?

If the required framework is not installed, your application won't run, so checking if the framework is installed from within your app is checking something you already know to be true.

If you want to check that the framework is installed, you really need to do this from within a a bootstrapper exe either written in a .NET version you know will exist (like .NET 2 as it gets installed on the machine with the OS) or some other language like C++.

You can check in the registry (HKLM\SOFTWARE\Microsoft\NET Framework Setup\NDP) to see what frameworks are installed. This can easily be done in C# or any other language.

C# code something like this:

var baseKeyName = @"SOFTWARE\Microsoft\NET Framework Setup\NDP";
var installedFrameworkVersions = Registry.LocalMachine.OpenSubKey(baseKeyName);

var versionNames = installedFrameworkVersions.GetSubKeyNames();

If you are deploying often for limited set of users, you should check out ClickOnce (http://msdn.microsoft.com/en-us/library/t71a733d(v=vs.80).ASPX)

It is a bit like Windows Installer, but simplified a lot. From the user perspective application will look like desktop icon. When you click on the icon, it will automatically check all the requirements and install the latest version of your software if it has been updated. This way you can ensure that users always has the required framework installed, as well users always will use the latest version.

To Check .net frame work version follow http://support.microsoft.com/kb/318785

I got at task to extend en existing WinForm application to make a check weather or not the required .NET Framework (fx. 3.5) is installed

This doesn't makes sense, because it will not run if required component is missing.

As already mentioned, typically installer is responsible for the job to check for components, interrupt installation, download and install them.

But in any case, nothing prevents you to make bootstrapper, which will be able run in majority of times (aka required less/none components) and which will run your .net application. It can be organized in multiple ways: exe + exe (often renamed), exe + dll, exe + embedded resource (extract and run), etc...

If you think to use MS Visual C++ to make bootstrapper, then here is a bad new for you: it required component to be installed (yes, that's stupid, but it needs Microsoft Visual C++ 200x redistributable). So it would be again a job for installer, or you will have to learn how to write C/C++ pure WinAPI software, which is a pain, but doable.

To example, this is a check for .Net Framework 4.0

// check if net framework 4.0 or higher is installed
HKEY hkey;
if (RegOpenKeyEx(HKEY_LOCAL_MACHINE, L"SOFTWARE\\Microsoft\\Net Framework Setup\\NDP\\v4", 0, KEY_READ, &hkey) != ERROR_SUCCESS)
    ; // some error
RegCloseKey(hkey);

Doesn't looks complicated? Then go ahead!

You are right about the program not running if the required .NET version is not installed. You could use a bootstrapper, as others have said. You could simply let the application fail at runtime with the default message box that appears whining about the missing dependency.

Another valid approach would be hosting the CLR yourself. Here:

• Hosting the Common Language Runtime
• CLR Hosting - Customizing the CLR

It's mostly like a bootstrapper too, but instead of launching the second (.NET-dependent) application it just loads it into the same process (if available).

You could create a batch (.bat) file to verify the existence of the .NET Framework folders:

IF EXIST C:\WINDOWS\Microsoft.NET\Framework[framework version here] (goto launchprogram) else goto errormessage