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How to: Open and Append to a Log File

StreamWriter and StreamReader write characters to and read characters from streams. The following code example opens the log.txt file for input, or creates the file if it does not already exist, and appends information to the end of the file. The contents of the file are then written to standard output for display. As an alternative to this example, the information could be stored as a single string or string array, and the WriteAllText or WriteAllLines method could be used to achieve the same functionality.


Visual Basic users may choose to use the methods and properties provided by the My.Application.Log or My.Computer.FileSystem objects for creating or writing to log files. For more information, see My.Application.Log Object and My.Computer.FileSystem Object.

using System;
using System.IO;
class DirAppend
    public static void Main(String[] args)
        using (StreamWriter w = File.AppendText("log.txt"))
            Log ("Test1", w);
            Log ("Test2", w);
            // Close the writer and underlying file.
        // Open and read the file.
        using (StreamReader r = File.OpenText("log.txt"))
            DumpLog (r);
    public static void Log (String logMessage, TextWriter w)
        w.Write("\r\nLog Entry : ");
        w.WriteLine("{0} {1}", DateTime.Now.ToLongTimeString(),
        w.WriteLine("  :");
        w.WriteLine("  :{0}", logMessage);
        w.WriteLine ("-------------------------------");
        // Update the underlying file.
    public static void DumpLog (StreamReader r)
        // While not at the end of the file, read and write lines.
        String line;
        while ((line=r.ReadLine())!=null)