Program

using System;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            string rootDir = "";

            rootDir = Directory.GetDirectoryRoot("D:/Sample/dir2/Green color");
            Console.WriteLine("Root Directory is : ("+rootDir+")");

            rootDir = Directory.GetDirectoryRoot("D:/Sample/dir2/Blue color");
            Console.WriteLine("Root Directory is : (" + rootDir + ")");

            rootDir = Directory.GetDirectoryRoot("C:/Windows/Help/Windows");
            Console.WriteLine("Root Directory is : (" + rootDir + ")");

        }
    }
}

Output

Root Directory is : (D:\)
Root Directory is : (D:\)
Root Directory is : (C:\)

Note: In above program, we need to remember, when we use "Directory" class, System.IO namespace must be included in the program.