Get Current .NET CLR version at runtime?

Check out the System.Environment.Version property.
https://docs.microsoft.com/en-us/dotnet/api/system.environment.version

Since .NET 4.5 you could just use System.Environment.Version (it would only return 4.0.{something}, allowing you to verify that you're "at least" on 4.0 but not telling you which actual version is available unless you can map a full list of build numbers in).

In .NET Core, and starting in .NET 4.7.1 of Framework, you can check System.Runtime.InteropServices.RuntimeInformation.FrameworkDescription

It returns a string with either: ".NET Core", ".NET Framework", or ".NET Native" before the version number -- so you have a little parsing to do if you just want the number.