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

Article
11/16/2012

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

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}

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

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

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).

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

To parse a text file with multiple formats

Example

Robust Programming

See Also

Tasks

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Reference

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