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How to: read from fixed-width text files in Visual Basic

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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

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:

See also