how to generate sequence number using C# window application? please help!!!!!!!

using System.IO;

private int GenerateSequenceNumber()
{
    string file2 = "test.txt";
    FileInfo fiRead = new FileInfo(file2);
    string input = string.Empty;
    int num = 1;
    if (fiRead.Exists)
    {
        input = File.ReadAllText(file2);
    }
    if (input != string.Empty)
    {
        num = Convert.ToInt32(input);
    }
    File.WriteAllText(file2, (num + 1).ToString());            
    return num;
}