Program

using System;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            String []dirs = Directory.GetDirectories("D:/Sample");

            Console.WriteLine("Sub directories are:");
            for (int i = 0; i < dirs.Length; i++)
            {
                Console.WriteLine("\t"+dirs[i]);
            }

        }
    }
}

Output

Sub directories are:
        D:/Sample\Blue Color
        D:/Sample\Blue Whale
        D:/Sample\Green color
        D:/Sample\Green vegetable
        D:/Sample\Red color

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