Small C# console application to show the currently installed .NET Framework version and Operating System.

using Microsoft.Win32;

namespace GetDotNetVersion.Console
{
    public static class GetDotNetVersion
    {
        public static void Main()
        {
            GetOsProductName();
            Get45PlusFromRegistry();
        }

        private static void Get45PlusFromRegistry()
        {
            using (var ndpKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry32)
                .OpenSubKey(@"SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full\"))
            {
                System.Console.WriteLine(ndpKey?.GetValue("Release") != null
                    ? $".NET Framework Version: {CheckFor45PlusVersion((int) ndpKey.GetValue("Release"))}"
                    : ".NET Framework Version 4.5 or later is not detected.");
            }

            // Checking the version using >= enables forward compatibility.
            string CheckFor45PlusVersion(int releaseKey)
            {
                if (releaseKey >= 528040)
                    return "4.8 or later";
                if (releaseKey >= 461808)
                    return "4.7.2";
                if (releaseKey >= 461308)
                    return "4.7.1";
                if (releaseKey >= 460798)
                    return "4.7";
                if (releaseKey >= 394802)
                    return "4.6.2";
                if (releaseKey >= 394254)
                    return "4.6.1";
                if (releaseKey >= 393295)
                    return "4.6";
                if (releaseKey >= 379893)
                    return "4.5.2";
                if (releaseKey >= 378675)
                    return "4.5.1";
                if (releaseKey >= 378389)
                    return "4.5";
                // This code 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";
            }
        }

        private static void GetOsProductName()
        {
            var productName = "";

            using (var reg = Registry.LocalMachine.OpenSubKey(@"SOFTWARE\Microsoft\Windows NT\CurrentVersion"))
            {
                if (reg != null) productName = reg.GetValue("ProductName") as string;
            }

            System.Console.WriteLine($"Operating System: {productName}");
        }
    }
}