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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 Log class or FileSystem class for creating or writing to log files.

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.
        End Using
        ' Open and read the file.
        Using r As StreamReader = File.OpenText("log.txt")
        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(), _
        w.WriteLine("  :")
        w.WriteLine("  :{0}", logMessage)
        w.WriteLine ("-------------------------------")
        ' Update the underlying file.
    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)
            line = r.ReadLine()
        End While
    End Sub
End Class

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