How to determine the clients .NET framework version in a web application?

You can use the Request.Browser.ClrVersion property to get the client's highest .NET version and Request.Browser.GetClrVersions() method to get all the installed .NET versions.

These methods simply parse the Request.ServerVariables("HTTP_USER_AGENT") server variable.

But please note that a browser (or user or hacker) may put anything he wishes in the string, so you won't have 100% accuracy.

I think you should do something like the following msdn article suggests. It uses java script to do the detection of .NET Framework.

One way could be to get the referenced assemblies list from current assembly. And then look for mscorlib.dll (or any other .net assembly that you are sure is loaded) and get the version of that assembly. This way you would know the version of framework installed.

try this code:

Version version = null;
AssemblyName[] names = Assembly.GetExecutingAssembly().GetReferencedAssemblies();
foreach (AssemblyName name in names)
{
      if (name.Name == "mscorlib")
      {
            version = name.Version;
      }
}

This all depends on the availability of the assembly that you choose to get the version from.

Or have a look at this CodeProject article. In this article he/she gives reference to another article by Junfeng Zhang which uses unmanaged code to determine CLR version.