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.

Note

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

Imports System
Imports System.IO

Class DirAppend
    Public Shared Sub Main()
        Using w As StreamWriter = File.AppendText("log.txt")
            Log("Test1", w)
            Log("Test2", w)
            ' Close the writer and underlying file.
            w.Close()
        End Using
        ' Open and read the file.
        Using r As StreamReader = File.OpenText("log.txt")
            DumpLog(r)
        End Using
    End Sub

    Public Shared Sub Log(logMessage As String, w As TextWriter)
        w.Write(vbCrLf + "Log Entry : ")
        w.WriteLine("{0} {1}", DateTime.Now.ToLongTimeString(), _
            DateTime.Now.ToLongDateString())
        w.WriteLine("  :")
        w.WriteLine("  :{0}", logMessage)
        w.WriteLine ("-------------------------------")
        ' Update the underlying file.
        w.Flush()
    End Sub

    Public Shared Sub DumpLog(r As StreamReader)
        ' While not at the end of the file, read and write lines.
        Dim line As String
        line = r.ReadLine()
        While Not (line Is Nothing)
            Console.WriteLine(line)
            line = r.ReadLine()
        End While
        r.Close()
    End Sub
End Class
using System;
using System.IO;

class DirAppend
{
    public static void Main()
    {
        using (StreamWriter w = File.AppendText("log.txt"))
        {
            Log("Test1", w);
            Log("Test2", w);
            // Close the writer and underlying file.
            w.Close();
        }
        // 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(),
            DateTime.Now.ToLongDateString());
        w.WriteLine("  :");
        w.WriteLine("  :{0}", logMessage);
        w.WriteLine ("-------------------------------");
        // Update the underlying file.
        w.Flush();
    }

    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)
        {
            Console.WriteLine(line);
        }
        r.Close();
    }
}
using namespace System;
using namespace System::IO;

ref class DirAppend
{
public:
    static void Main()
    {
        StreamWriter^ w = File::AppendText("log.txt");
        Log("Test1", w);
        Log("Test2", w);
        // Close the writer and underlying file.
        w->Close();
        // Open and read the file.
        StreamReader^ r = File::OpenText("log.txt");
        DumpLog(r);
        r->Close();
    }

    static void Log(String^ logMessage, TextWriter^ w)
    {
        w->Write("\r\nLog Entry : ");
        w->WriteLine("{0} {1}", DateTime::Now.ToLongTimeString(),
            DateTime::Now.ToLongDateString());
        w->WriteLine("  :");
        w->WriteLine("  :{0}", logMessage);
        w->WriteLine ("-------------------------------");
        // Update the underlying file.
        w->Flush();
    }

    static void DumpLog(StreamReader^ r)
    {
        // While not at the end of the file, read and write lines.
        String^ line;
        while ((line = r->ReadLine()) != nullptr)
        {
            Console::WriteLine(line);
        }
        r->Close();
    }
};

int main()
{
    DirAppend::Main();
}

See Also

Tasks

How to: Create a Directory Listing

How to: Read and Write to a Newly Created Data File

How to: Read Text from a File

How to: Write Text to a File

How to: Read Characters from a String

How to: Write Characters to a String

Reference

StreamWriter

StreamReader

File.AppendText

File.OpenText

StreamReader.ReadLine

Concepts

Basic File I/O