Is there an easy way to check the .NET Framework version?

Something like this should do it. Just grab the value from the registry

For .NET 1-4:

Framework is the highest installed version, SP is the service pack for that version.

RegistryKey installed_versions = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\NET Framework Setup\NDP");
string[] version_names = installed_versions.GetSubKeyNames();
//version names start with 'v', eg, 'v3.5' which needs to be trimmed off before conversion
double Framework = Convert.ToDouble(version_names[version_names.Length - 1].Remove(0, 1), CultureInfo.InvariantCulture);
int SP = Convert.ToInt32(installed_versions.OpenSubKey(version_names[version_names.Length - 1]).GetValue("SP", 0));

For .NET 4.5+ (from official documentation):

using System;
using Microsoft.Win32;


...


private static void Get45or451FromRegistry()
{
    using (RegistryKey ndpKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry32).OpenSubKey("SOFTWARE\\Microsoft\\NET Framework Setup\\NDP\\v4\\Full\\")) {
        int releaseKey = Convert.ToInt32(ndpKey.GetValue("Release"));
        if (true) {
            Console.WriteLine("Version: " + CheckFor45DotVersion(releaseKey));
        }
    }
}


...


// Checking the version using >= will enable forward compatibility,  
// however you should always compile your code on newer versions of 
// the framework to ensure your app works the same. 
private static string CheckFor45DotVersion(int releaseKey)
{
    if (releaseKey >= 528040) {
        return "4.8 or later";
    }
    if (releaseKey >= 461808) {
        return "4.7.2 or later";
    }
    if (releaseKey >= 461308) {
        return "4.7.1 or later";
    }
    if (releaseKey >= 460798) {
        return "4.7 or later";
    }
    if (releaseKey >= 394802) {
        return "4.6.2 or later";
    }
    if (releaseKey >= 394254) {
        return "4.6.1 or later";
    }
    if (releaseKey >= 393295) {
        return "4.6 or later";
    }
    if (releaseKey >= 393273) {
        return "4.6 RC or later";
    }
    if ((releaseKey >= 379893)) {
        return "4.5.2 or later";
    }
    if ((releaseKey >= 378675)) {
        return "4.5.1 or later";
    }
    if ((releaseKey >= 378389)) {
        return "4.5 or later";
    }
    // This line should never execute. A non-null release key should mean 
    // that 4.5 or later is installed. 
    return "No 4.5 or later version detected";
}