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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:

See Also

Show: