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How to: Open and append to a log file

  • Artikkeli

StreamWriter and StreamReader write characters to and read characters from streams. The following code example opens the log.txt file for input, or creates it if it doesn't exist, and appends log information to the end of the file. The example then writes the contents of the file to standard output for display.

As an alternative to this example, you could store the information as a single string or string array, and use the File.WriteAllText or File.WriteAllLines method to achieve the same functionality.

Huomautus

Visual Basic users may choose to use the methods and properties provided by the Log class or FileSystem class for creating or writing to log files.

Example

using System;
using System.IO;

class DirAppend
{
    public static void Main()
    {
        using (StreamWriter w = File.AppendText("log.txt"))
        {
            Log("Test1", w);
            Log("Test2", w);
        }

        using (StreamReader r = File.OpenText("log.txt"))
        {
            DumpLog(r);
        }
    }

    public static void Log(string logMessage, TextWriter w)
    {
        w.Write("\r\nLog Entry : ");
        w.WriteLine($"{DateTime.Now.ToLongTimeString()} {DateTime.Now.ToLongDateString()}");
        w.WriteLine("  :");
        w.WriteLine($"  :{logMessage}");
        w.WriteLine ("-------------------------------");
    }

    public static void DumpLog(StreamReader r)
    {
        string line;
        while ((line = r.ReadLine()) != null)
        {
            Console.WriteLine(line);
        }
    }
}
// The example creates a file named "log.txt" and writes the following lines to it,
// or appends them to the existing "log.txt" file:

// Log Entry : <current long time string> <current long date string>
//  :
//  :Test1
// -------------------------------

// Log Entry : <current long time string> <current long date string>
//  :
//  :Test2
// -------------------------------

// It then writes the contents of "log.txt" to the console.

See also