Creating and appending text to txt file in VB.NET

Try this:

Dim strFile As String = "yourfile.txt"
Dim fileExists As Boolean = File.Exists(strFile)
Using sw As New StreamWriter(File.Open(strFile, FileMode.OpenOrCreate))
    sw.WriteLine( _
        IIf(fileExists, _
            "Error Message in  Occured at-- " & DateTime.Now, _
            "Start Error Log for today"))
End Using

You didn't close the file after creating it, so when you write to it, it's in use by yourself. The Create method opens the file and returns a FileStream object. You either write to the file using the FileStream or close it before writing to it. I would suggest that you use the CreateText method instead in this case, as it returns a StreamWriter.

You also forgot to close the StreamWriter in the case where the file didn't exist, so it would most likely still be locked when you would try to write to it the next time. And you forgot to write the error message to the file if it didn't exist.

Dim strFile As String = "C:\ErrorLog_" & DateTime.Today.ToString("dd-MMM-yyyy") & ".txt"
Dim sw As StreamWriter
Try
   If (Not File.Exists(strFile)) Then
      sw = File.CreateText(strFile)
      sw.WriteLine("Start Error Log for today")
   Else
      sw = File.AppendText(strFile)
   End If
   sw.WriteLine("Error Message in  Occured at-- " & DateTime.Now)
   sw.Close()
Catch ex As IOException
   MsgBox("Error writing to log file.")
End Try

Note: When you catch exceptions, don't catch the base class Exception, catch only the ones that are releveant. In this case it would be the ones inheriting from IOException.

This should work for you without changing program logic (by not outputting "Start error" on the top of each file) like the other answers do :) Remember to add exception handling code.

Dim filePath As String = String.Format("C:\ErrorLog_{0}.txt", DateTime.Today.ToString("dd-MMM-yyyy"))

Dim fileExists As Boolean = File.Exists(filePath)

Using writer As New StreamWriter(filePath, True)
    If Not fileExists Then
        writer.WriteLine("Start Error Log for today")
    End If
    writer.WriteLine("Error Message in  Occured at-- " & DateTime.Now)
End Using

Don't check File.Exists() like that. In fact, the whole thing is over-complicated. This should do what you need:

Dim strFile As String = $@"C:\ErrorLog_{DateTime.Today:dd-MMM-yyyy}.txt"
File.AppendAllText(strFile, $"Error Message in  Occured at-- {DateTime.Now}{Environment.NewLine}")

Got it all down to two lines of code :)