How to: Read From Text Files with Multiple Formats in Visual Basic

The TextFieldParser object provides a way to easily and efficiently parse structured text files, such as logs. You can process a file with multiple formats by using the PeekChars method to determine the format of each line as you parse through the file.

To parse a text file with multiple formats

1. Define the expected format and the format used when an error is reported.

   Dim StdFormat As Integer()= {5,10,11,-1}
   Dim ErrorFormat As Integer() = {5,5,-1}
   

2. Create a new TextFieldParser object, defining the width and format.

   Using MyReader As New _
   Microsoft.VisualBasic.FileIO.TextFieldParser("C:\testfile.txt")
   MyReader.TextFieldType = FileIO.FieldType.FixedWidth
   

3. Loop through the rows, testing for format before reading.

   Dim CurrentRow As String()
   While Not MyReader.EndOfData
      Try 
         Dim RowType As String = MyReader.PeekChars(3)
         If String.Compare(RowType, "Err") = 0 Then 
            ' If this line describes an error, the format of  
            ' the row will be different.
            MyReader.SetFieldWidths(ErrorFormat)
            CurrentRow = MyReader.ReadFields
            MyReader.SetFieldWidths(StdFormat)
                       Else 
            'Otherwise parse the fields normally
            CurrentRow = MyReader.ReadFields
            For Each newString As String In CurrentRow
               My.Computer.FileSystem.WriteAllText _
               ("newFile.txt", newString, True)
             Next 
      End If
   

4. Write errors to the console.

   Catch ex As _
         Microsoft.VisualBasic.FileIO.MalformedLineException
            MsgBox("Line " & ex.Message & " is invalid.")
         End Try 
      End While 
   End Using
   

Example

This example reads from the file testfile.txt.

Dim StdFormat As Integer() = {5, 10, 11, -1}
Dim ErrorFormat As Integer() = {5, 5, -1}
Using MyReader As New _
Microsoft.VisualBasic.FileIO.TextFieldParser("C:\testfile.txt")
   MyReader.TextFieldType = FileIO.FieldType.FixedWidth
   MyReader.FieldWidths = StdFormat
   Dim CurrentRow As String()
      While Not MyReader.EndOfData
         Try 
            Dim RowType As String = MyReader.PeekChars(3)
            If String.Compare(RowType, "Err") = 0 Then 
               ' If this line describes an error, the format of the row will be different.
               MyReader.SetFieldWidths(ErrorFormat)
               CurrentRow = MyReader.ReadFields
               MyReader.SetFieldWidths(StdFormat)
            Else 
               ' Otherwise parse the fields normally
               CurrentRow = MyReader.ReadFields
               For Each newString As String In CurrentRow
                  My.Computer.FileSystem.WriteAllText("newFile.txt", newString, True)
               Next 
            End If 
         Catch ex As Microsoft.VisualBasic.FileIO.MalformedLineException
            MsgBox("Line " & ex.Message & " is invalid.  Skipping")
         End Try 
      End While 
End Using

Robust Programming

The following conditions may cause an exception:

• A row cannot be parsed using the specified format (MalformedLineException). The exception message specifies the line causing the exception, while the TextFieldParser.ErrorLine Property is assigned to the text contained in the line.

• The specified file does not exist (FileNotFoundException).

• A partial-trust situation in which the user does not have sufficient permissions to access the file. (SecurityException).

• The path is too long (PathTooLongException).

• The user does not have sufficient permissions to access the file (UnauthorizedAccessException).

See Also

Tasks

How to: Read From Comma-Delimited Text Files in Visual Basic

How to: Read From Fixed-width Text Files in Visual Basic

Concepts

Parsing Text Files with the TextFieldParser Object

Reference

TextFieldParser Object

TextFieldParser.PeekChars Method

MalformedLineException

My.Computer.FileSystem.WriteAllText Method

TextFieldParser.EndOfData Property

TextFieldParser.TextFieldType Property