Getting the .NET Framework directory path

How can I obtain the .NET Framework directory path inside my C# application?

The folder that I refer is "C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727"

The path to the installation directory of the CLR active for the current .NET application can be obtained by using the following method:

System.Runtime.InteropServices.RuntimeEnvironment.GetRuntimeDirectory()

I would strongly advice against reading the registry directly. For example, when a .NET application is running in 64bit systems, the CLR can either be loaded from "C:\Windows\Microsoft.NET\Framework64\v2.0.50727" (AnyCPU, x64 compilation targets) or from "C:\Windows\Microsoft.NET\Framework\v2.0.50727" (x86 compilation target). Reading registry will not tell you which one of the two directories was used by the current CLR.

Another important fact is that "the current CLR" will be "2.0" for .NET 2.0, .NET 3.0 and .NET 3.5 applications. This means that the GetRuntimeDirectory() call will return 2.0 directory even within .NET 3.5 applications (that load some of their assemblies from 3.5 directory). Depending on your interpretation of the term ".NET Framework directory path", GetRuntimeDirectory might not be the information you are looking for ("CLR directory" versus "directory from which 3.5 assemblies are coming from").

An easier way is to include the Microsoft.Build.Utilities assembly and use

using Microsoft.Build.Utilities;
ToolLocationHelper.GetPathToDotNetFramework(
        TargetDotNetFrameworkVersion.VersionLatest);

You can grab it from the Windows Registry:

using System;
using Microsoft.Win32;

// ...

public static string GetFrameworkDirectory()
{
  // This is the location of the .Net Framework Registry Key
  string framworkRegPath = @"Software\Microsoft\.NetFramework";

  // Get a non-writable key from the registry
  RegistryKey netFramework = Registry.LocalMachine.OpenSubKey(framworkRegPath, false);

  // Retrieve the install root path for the framework
  string installRoot = netFramework.GetValue("InstallRoot").ToString();

  // Retrieve the version of the framework executing this program
  string version = string.Format(@"v{0}.{1}.{2}\",
    Environment.Version.Major, 
    Environment.Version.Minor,
    Environment.Version.Build); 

  // Return the path of the framework
  return System.IO.Path.Combine(installRoot, version);     
}

Source

For .NET Framework versions >= 4.5, an official way from MSDN:

internal static class DotNetFrameworkLocator
{
    public static string GetInstallationLocation()
    {
        const string subkey = @"SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full\";
        using (var ndpKey = RegistryKey.OpenBaseKey(RegistryHive.LocalMachine, RegistryView.Registry32).OpenSubKey(subkey))
        {
            if (ndpKey == null)
                throw new Exception();

            var value = ndpKey.GetValue("InstallPath") as string;
            if (value != null)
                return value;
            else
                throw new Exception();
        }
    }
}

Read value of the [HKLM]\Software\Microsoft.NetFramework\InstallRoot key - you will get "C:\WINDOWS\Microsoft.NET\Framework". Then append with desired framework version.