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

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11/24/2013

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

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")
```
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)
```

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

Close the While and Using blocks with End While and End Using.
```
End While 
End Using
```

Example

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

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 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).

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

To parse a fixed-width text file

Example

Robust Programming

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