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C# application host version detection when no .NET runtime is installed

I have an application that relies on .NET 6 runtime. If I don't have any .NET runtimes installed on my machine, not only the one my application needs but also without other versions of .net runtime, I could see some messages prompted when execute it.
Does anyone have any insight about how the detection work or who gives the prompted information? Is it done by the OS?
Thanks!

Windows 10 and 11 comes pre-installed with .NET Framework.
A .NET 6 application can be released as a self-contained application that includes the required runtimes to run (if it is not on the machine) as suggested here; https://stackoverflow.com/a/72630528/1165173
https://learn.microsoft.com/en-us/dotnet/core/install/how-to-detect-installed-versions?pivots=os-windows

does updating .net version when compiling affects client dependencies?

If I update from .net 4.5 to .net 4.8 when i compile my project. Does that introduce a risk that the clients cant run the dlls?
Do all users have to upgrade to .net 4.8 on their machines or are the .net version backwards compatible? They have all been able to run the app when it was compiled with .net 4.5.
What im trying to ask is if i risk that some clients wont be able to run the app by upgrading the Target Framework when building it.
This is from 4.5 to 4.8.

Does that introduce a risk that the clients cant run the dlls?
Yes
Do all users have to upgrade to .net 4.8 on their machines
Yes, if they do not already have it.
are the .net version backwards compatible?
Yes, they are backwards compatible, meaning a user with .net 4.8 can run programs written for .net 4.5 with no issue. Not the other way around.
What i'm trying to ask is if i risk that some clients wont be able to run the app by upgrading the Target Framework when building it.
They should have .net 4.8 installed since it is bundled with windows 10 1903 and later. See versions and dependencies. All older versions of windows 10 is end of life. The only exceptions are windows server 2019/2016.
Even if your client lacks .Net 4.8, windows should simply ask if they want to install .net 4.8, so it should be very easy to do as long as the users have permission to install things on their computers.

Getting error if trying to download my standalone app: Unable to find a version of the runtime to run this application

I use VS 2010 C# Express
The file is digitally signed (signed/verified with signtool)
Target framework: .NET Framework 4 Client Profile
.NET Framework 4 Client Profile is installed
I'm uploading the exe to a simple static html page
If I (or anyone) click on the download link get the message above
Right-click -> Save Target As... works OK
I can run the file without issues after downloading

May I ask why you are targeting .NET 4.0 Client Platform specifically? Why not go ahead and target to one of the 4.5 versions? Is there a requirement to target a discontinued release of .NET?
Within that link and this one under the 'Additional Information' section, there are statements that indicate an 'in place update' to .NET 4.0 is made to migrate an already installed .NET 4.0 Client Platform to the .NET 4.0 Full Install -- there doesn't appear to be anything written to indicate that 4.0 gets installed by the 4.5 installer, if it's not already installed on the system.
Therefore, all machines with .NET 4.5 installed would have replaced the mentioned 'target framework' of .NET that you're building with something that no longer exists on those machines ... which I believe might account for the popup error received. Can you, at least, try to target one of the 4.5 versions of .NET and update the executable to download on your server? Try again and see if the error goes away? If it does, then the problem that you are facing pertains to .NET Client Platform not (or longer due to 4.5 update) being installed on the client machines.
If that's the case and .NET 4.0 needs to be targeted, then you might need to target the Full 4.0 version instead of the Client Profile version. Or just move to one of the 4.5 versions.

I'd venture to guess that the problem is not within the executable itself but somewhere in the delivery of it into the web page or in the delivery of it back down to the end user; especially considering the program will run without issue once it's downloaded.
Have you completely vetted that process? You mention 'simple static html page' - elaborate on that a bit and you might get more suggestions that will lead you toward a proper solution. Web server used? Technology involved? Personal machine running apache or the like? or some website generator? or something like Wordpress? or some other web hosting company and their products? If the problem lies therein, then further details would be nice to have to help you deduce what is causing issue.
As I'm sure you're aware, that particular error message appears to be presented mostly to products that utilize products created with or included libraries requiring .NET 2.0, 3.0, or 3.5 - since they are (by default) no longer pre-installed on newer systems released with Windows 8+ ... a simple test to rule out the possibility would be to simply install the .NET 3.5 installer and see if the problem magically goes away - if it does, then you may need to do a bit of further research on the 'delivery mechanism' - read, web server used - technology it requires - etc ...
If that's way off ... perhaps this is an x86 vs x64 problem? 32bit app running on 64bit OS? something along those lines? A simple test would be to create 2 executables: one that is configured to build all output as x86 and one to build all output as x64 - see if both behave the same or if there is a difference, and move forward from there.
I believe that in this situation, you will need to play around and run a few tests before getting to the bottom of it. Good luck to you.

Filedialog CustomPlaces confusion

I faced a strange behavior using the following code:
FileDialog openFileDialog1;
// ...
openFileDialog1.CustomPlaces.Add(#"C:\whatever\");
This compiles with no errors using .NET framework 2.0.
The code runs well under Windows 7.
But under Windows XP I get the following error at runtime:
System.MissingMethodException: Method not found: 'System.Windows.Forms.FileDialogCustomPlacesCollection System.Windows.Forms.FilaDialog.get_CustomPlaces()'.
Trying to figure out the problem tells me:
Visual Studio 2005 help don't know about the CustomPlaces property of FileDialog
MSDN says that this property exists only since framework 3.5 - and "On Windows XP, this property does not have any effect."
Visual Studio 2005 intellisense offers me the exsistence of the property (so I thought using this property is fine)
That obviously doesn't fit toghether.
I still want to use the code, so I'm trying to figure out how a do check before calling it:
if (...) {
openFileDialog1.CustomPlaces.Add(#"C:\whatever\");
}
My question is:
Is CustomPlaces not supported by .NET 2.0 or is is not supported by Windows XP?
How do I handle this correctly?
1) Do I have to check for the Windows version:
if (Environment.OSVersion.Version.Major >= 6) ...
2) Or do I have to check for the framework version:
if (Environment.Version.Major >= 4) ...
3) Or both, or else !?

It seems you have answered your own question in your post. The MSDN documentation clearly states that the FileDialog.CustomPlaces property exists only from .Net Framework 3.5 onwards. If your application is running on .Net Framework 2.0, then no, it's not supported. (Actually, looking at the .NET 4.5 MSDN documentation for the property, it seems it's supported as of .Net Framework 2.0 SP1.)
If you are using Visual Studio 2010 or higher, I would ensure that you retarget your project to compile with the .Net Framework 2.0 (RTM) if that's what framework your application will be running against. I noticed that you specifically mentioned VS 2005. If you are using VS 2005, I don't believe that has any re-targeting capabilities (if memory serves me correctly); VS2005 by default compiles against .NET Framework 2.0. Here's where things get interesting: an installation of .NET Framework 3.5 has .NET Framework 2.0 SP1 and .NET Framework 3.0 SP1 as installation pre-requisites. So, if your machine has only .NET Framework 2.0 and you want to install .NET Framework 3.5, then your .NET Framework 2.0 will be upgraded to .NET Framework 2.0 SP1 (and additionally, .NET Framework 3.0 SP1 will also be installed).
So what does this mean for you? Well, if you compile your program on a machine on which .NET Framework 2.0 SP1 or higher is installed, your program will compile just fine. Furthermore, if you run the program on a machine running .NET Framework 2.0 SP1 or higher, it will also run just fine. However, if you take this same program and run it on a machine which contains only the vanilla .NET Framework 2.0, you will get the error you see above because that method/property is not supported in the libraries included with the .NET Framework 2.0.
The other side of the story is the part of the MSDN documentation that says that while FileDialog.CustomPlaces exists from .Net Framework 2.0 SP1 and onwards, and that while that version of the Framework may be installed on Windows XP, that on Windows XP calling the property has no effect (i.e. it's a null-op, not supported by the OS). So no error should occur, but you will also see that whatever you tried to add to the CustomPlaces collection would not show up when you run the application on Windows XP. Again, looking at the updated documentation as of .Net Framework 4.5.x (see the link above), it clearly states that the lowest supported client operating system platform supported is Windows Vista SP2. So in all likelihood, you're out of luck when it comes to Windows XP.
If you want to do an OS version check, I would advise you to do the following check:
if (Environment.OSVersion.Version.Major > 5 &&
Environment.OSVersion.Version.Minor >= 0 &&
Environment.OSVersion.ServicePack == "Service Pack 2")
{
// Add CustomPlace here...
}
Note that the check above will not allow you to add the FileDialog.CustomPlaces for Windows Server 2008 (even though it is supported—because through .NET, you're not able to check the ProductTypeID property). Furthermore, FileDialog.CustomPlaces is not supported on the Server Core mode of Windows until Windows Server 2008 R2 SP1 (but the code above will allow you to attempt to add the custom place, but like Windows XP, it will fail silently). In order to make a determination for these versions of Windows, you'll need to use a bit of PInvoke to access the Win32 API's GetVersionEx method and OSVERSIONINFOEX structure located in kernel32.dll as shown here and/or here.
However, generally speaking, it's not a good idea to do operating system version checks. They are notoriously difficult (especially with older operating systems). What you really need to do is to perform a .NET Framework version check. Your application apparently requires, at a minimum, .NET Framework 2.0 SP1. If you have an installer for your program, I would build in this check to your installer and optionally provide the ability to install the .NET Framework 2.0 SP1 as part of the installation of your program.
If you're not using an installer to distribute your program, then you should perform a .NET Framework version check in your application prior to attempting to add the FileSystemCustomPlace to the CustomPlaces collection to prevent the error from occurring; however, doing so will require your users with versions of Windows Vista and later to run the application with elevated permissions. Aaron Stebner has a great blog post on how to determine the .NET Framework installations that are available on your machine with example code. There is also a C# implementation of this code provided by a CodeProject.com user here.

Why does referncing earlier .net versions work on my local machine but not on another?

I have winforms application. When I look at the references they are all .net 4 (same as the application) apart from one which says v2.0. I can run the app locally and use functionality from the .net components but as soon as I take the app to another machine it wont fire up. Apparently this hapens when references cant be found.
Im very confused as to why this would work locally. I have checked the GAC and only .net 2.0 versions exist for the referenced dlls.?..
What are the rules around rererencing dlls from older versions on .net?

It is perfectly fine to have the two versions of the framework running on the same machine. Are both .NET 2 and .NET 4 installed on the second machine you are testing on? I suspect that they are not.

Yes you can reference .Net 2.0 dlls from .Net 4.0 project.
You might be referencing only .Net 2.0 dlls but your target framework might be .Net 4.0, which might not be available on your other machine.
To correct this:
Open your project properties.
Set Target framework to .Net Framework 2.0 or 4.0.
If you set it to 4.0, you must have 4.0 installed on the machine you want it to run on.
The quickest way to see which versions are installed on a machine:
open VS command prompt and type clrver. (Ofcourse you'll need .Net SDK)
To see which framework your app uses type clrver
you can get for your application from task manager (in process tab, click View->select columns->check PID)
Starting .NET 4.0, your app can target multiple frameworks in different AppDomains in the same process.
Remember that the framework that your application is determined automatically, but you can override this default behaviours. One way to do this is what I described above.

Note that the GAC location has changed between .Net 2.0 and .Net 4.0. You most likely have both versions installed on your machine and you have looked at the 2.0 GAC.
The second machine most likely does not have 2.0 installed, hence it fails running.