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

Article
11/16/2012

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

The TextFieldType property defines whether it is a delimited file or one with fixed-width fields of text. To specify a field of variable width in a fixed-width file, define the field width as -1.

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 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).

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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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