C#

<br />
public class MyTrace <br />
{<br />
   private readonly string tracePath;<br />
   private StreamWriter writer;<br />
<br />
   public MyTrace( string filename )<br />
   {<br />
      this.tracePath = filename;<br />
   }<br />
<br />
   public bool Open( )<br />
   {<br />
      try<br />
         {<br />
           writer = new StreamWriter( tracePath );<br />
           return true;<br />
         }<br />
      catch<br />
         {<br />
           return false;<br />
         }<br />
    }<br />
   public void Log( string format, params object[] args )<br />
   {  <br />
       if ( writer != null )<br />
       {<br />
          string msg = string.Format( format, args );<br />
          writer.WriteLine( msg ); // note: this can be whatever write method you want<br />
       }<br />
       else<br />
          throw new Exception( "Trace File must be opened" );<br />
   }<br />
   public void Close( )<br />
   {<br />
       writer.Flush( );<br />
       writer.Close( );<br />
       writer = null;<br />
   }<br />
}<br />

using System;
using System.Threading;
using System.IO;

namespace Threaded
{
	public class ThreadingTextWriter
	{
		private string path = "";
		private string textToWrite = DateTime.Now.ToString();
		
		public string Path
		{
			get
			{
				return this.path;
			}
			set
			{
				this.path = value;
			}
		}
		public string TextToWrite
		{
			get
			{
				return this.textToWrite;
			}
			set
			{
				this.textToWrite = value;
			}
		}
		
		public void Write()
		{
			Thread.Sleep(1000);
			File.WriteAllText(path, this.textToWrite);
		}
		
		public ThreadingTextWriter(string path, string textToWrite)
		{
			this.path = path;
			this.textToWrite = textToWrite;
		}
	}
	
	public static class Program
	{
		public static void Main(string[] args)
		{
			ThreadingTextWriter ttw1 = new ThreadingTextWriter(@"output1.txt", @"Output from ThreadingTextWriter1");
			ThreadingTextWriter ttw2 = new ThreadingTextWriter(@"output2.txt", @"Output from ThreadingTextWriter2");
			
			// Create new thread
			Thread ttw2Thread = new Thread(new ThreadStart(ttw2.Write));
			
			// Start write from 2
			ttw2Thread.Start();
			
			// Write from 1
			ttw1.Write();
			
			// Wait to make sure that all threads have finished writing.
			Thread.Sleep(1000);
			
			Console.WriteLine(@"Completed write tests");
		}
	}
}

• I havn't locked the files so other processes may try and access them. To solve this you can use the FileStream constructor as you have done, this I think has an option for locking the file for read/write access, I just can't remember off the top of my head
• This uses .NET 2.0 (hence the File.WriteAllText) so this will need to be changed to suit you needs