int i = random; //your random number
string[] lines = File.ReadAllLines(filepath); //make this an instance variable to avoid reading the file over and over again
string randomLine = lines[i];
Console.WriteLine(randomLine);

// i have added the namespace System.IO;
           // System.IO.StreamReader
            StreamReader sr = new StreamReader("C:\\t.txt");

            string line = string.Empty;
            Random r = new Random();
            // the line count starts at 0
            int i = 0;
            // set the line we want between 0 and the maximum number
            int j = r.Next(0, 2);
           /// loop through all the lines skipping nulls.
            while ((line = sr.ReadLine()) != null)
            {
                /// the random number we stored in int j will match and we go the random line.
                if (i == j)
                {
                    label1.Text = line;
                }
                i++;
            }