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

The TextFieldParser object provides a way to easily and efficiently parse structured text files, such as logs.

The TextFieldType property defines whether the parsed file is a delimited file or one that has fixed-width fields of text. In a fixed-width text file, the field at the end can have a variable width. To specify that the field at the end has a variable width, define it to have a width less than or equal to zero.

To parse a fixed-width text file

  1. Create a new TextFieldParser. The following code creates the TextFieldParser named Reader and opens the file test.log.

    Using Reader As New Microsoft.VisualBasic.
        FileIO.TextFieldParser("C:\TestFolder\test.log")
    
  2. Define the TextFieldType property as FixedWidth, defining the width and format. The following code defines the columns of text; the first is 5 characters wide, the second 10, the third 11, and the fourth is of variable width.

    Reader.TextFieldType =
    Microsoft.VisualBasic.FileIO.FieldType.FixedWidth
    Reader.SetFieldWidths(5, 10, 11, -1)
    
  3. Loop through the fields in the file. If any lines are corrupted, report an error and continue parsing.

    Dim currentRow As String()
       While Not Reader.EndOfData
          Try
             currentRow = Reader.ReadFields()
             Dim currentField As String 
             For Each currentField In currentRow
                MsgBox(currentField)
             Next 
          Catch ex As Microsoft.VisualBasic.
                      FileIO.MalformedLineException
             MsgBox("Line " & ex.Message &
             "is not valid and will be skipped.")
     End Try
    
  4. Close the While and Using blocks with End While and End Using.

       End While 
    End Using
    

This example reads from the file test.log.

Using Reader As New Microsoft.VisualBasic.FileIO.
   TextFieldParser("C:\TestFolder\test.log")

   Reader.TextFieldType =
      Microsoft.VisualBasic.FileIO.FieldType.FixedWidth
   Reader.SetFieldWidths(5, 10, 11, -1)
   Dim currentRow As String()
   While Not Reader.EndOfData
      Try
         currentRow = Reader.ReadFields()
         Dim currentField As String 
         For Each currentField In currentRow
            MsgBox(currentField)
         Next 
      Catch ex As Microsoft.VisualBasic.FileIO.MalformedLineException
         MsgBox("Line " & ex.Message &
         "is not valid and will be skipped.")
      End Try 
   End While 
End Using

The following conditions may cause an exception:

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