Get unknown exception while rewriting text file

So i have a function call classifier that basically check all text file in a directory, if there is any text file contains more than 50 line of words, then classify that text file into a type using other class called Text_Classification which i am sure its correct and error free. After classification, i will need to clean that text file and write a "Lines" as a first new line on that text file.(this is for other class, so dont bother :) )
But i got an exception, which mean there is something wrong in the try{} block.
Any idea why?

private static void classifer(object state)
         {
 
             Console.WriteLine("Time to check if any log need to be classified");
 
             string[] filename = Directory.GetFiles(@"C:\Users\Visual Studio 2010\Projects\server_test\log");
             foreach (string textfile_name in filename)
             {
                 var lineCount = File.ReadAllLines(textfile_name).Length;
                 if (lineCount > 50)
                 {
                     Console.WriteLine("Start classifying 1 of the logs");
                     Text_Classification classifier = new Text_Classification(textfile_name, 1);
                     string type = classifier.passBack_type();  //this function passes back the type of the log(text file)
                     Console.WriteLine(type);   
 
                     try
                     {
                         TextWriter tw = new StreamWriter(textfile_name); //clean the text file
                         tw.WriteLine(" ");
                         tw.Close();
 
 
                         TextWriter tw2 = new StreamWriter(textfile_name, true);  //append <Lines> as the first new line on the text file
                         tw2.WriteLine("Lines");
                         tw2.Close();
                     }
                     catch {
                         Console.WriteLine("cant re-write txt");
                     }
 
                 }
 
             }
         }
 

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