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 Characters from a String
How to: Write Characters to a String