using System;
using System.IO;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main()
        {
            byte[] b2 = new byte[6];

            FileStream f1;
            FileStream f2;

            f1 = new FileStream("abc.txt", FileMode.Open, FileAccess.Read);
            f2 = new FileStream("xyz.txt", FileMode.Open, FileAccess.Write);

            if (f1.CanWrite)
                Console.WriteLine("Yes, It can write.");
            else
                Console.WriteLine("Yes, It can not write.");


            f1.Close();

            if (f2.CanWrite)
                Console.WriteLine("Yes, It can write.");
            else
                Console.WriteLine("Yes, It can not write.");

            f2.Close();
        }
    }
}

Output

Yes, It can not write.
Yes, It can write.

In above program, we need to remember, when we use "FileStream" class then we need to include System.IO namespace in the program.