Create File If File Does Not Exist

You can simply call

using (StreamWriter w = File.AppendText("log.txt"))

It will create the file if it doesn't exist and open the file for appending.

Edit:

This is sufficient:

string path = txtFilePath.Text;               
using(StreamWriter sw = File.AppendText(path))
{
  foreach (var line in employeeList.Items)                 
  {                    
    Employee e = (Employee)line; // unbox once
    sw.WriteLine(e.FirstName);                     
    sw.WriteLine(e.LastName);                     
    sw.WriteLine(e.JobTitle); 
  }                
}     

But if you insist on checking first, you can do something like this, but I don't see the point.

string path = txtFilePath.Text;               


using (StreamWriter sw = (File.Exists(path)) ? File.AppendText(path) : File.CreateText(path))                 
{                      
    foreach (var line in employeeList.Items)                     
    {                         
      sw.WriteLine(((Employee)line).FirstName);                         
      sw.WriteLine(((Employee)line).LastName);                         
      sw.WriteLine(((Employee)line).JobTitle);                     
    }                  
} 

Also, one thing to point out with your code is that you're doing a lot of unnecessary unboxing. If you have to use a plain (non-generic) collection like ArrayList, then unbox the object once and use the reference.

However, I perfer to use List<> for my collections:

public class EmployeeList : List<Employee>

or:

using FileStream fileStream = File.Open(path, FileMode.Append);
using StreamWriter file = new StreamWriter(fileStream);
// ...

You don't even need to do the check manually, File.Open does it for you. Try:

using (StreamWriter sw = new StreamWriter(File.Open(path, System.IO.FileMode.Append))) 
{

Ref: http://msdn.microsoft.com/en-us/library/system.io.filemode.aspx

Yes, you need to negate File.Exists(path) if you want to check if the file doesn't exist.

2021

Just use File.AppendAllText, which creates the file if it does not exist:

File.AppendAllText("myFile.txt", "some text");