How to: Read from text files with multiple formats in Visual Basic

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

1. Add a text file named testfile.txt to your project. Add the following content to the text file:

   Err 1001 Cannot access resource.
   Err 2014 Resource not found.
   Acc 10/03/2009User1 Administrator.
   Err 0323 Warning: Invalid access attempt.
   Acc 10/03/2009User2 Standard user.
   Acc 10/04/2009User2 Standard user.
   

2. Define the expected format and the format used when an error is reported. The last entry in each array is -1, therefore the last field is assumed to be of variable width. This occurs when the last entry in the array is less than or equal to 0.

   Dim stdFormat As Integer() = {5, 10, 11, -1}
   Dim errorFormat As Integer() = {5, 5, -1}
   

3. Create a new TextFieldParser object, defining the width and format.

   Using MyReader As New FileIO.TextFieldParser("..\..\testfile.txt")
    MyReader.TextFieldType = FileIO.FieldType.FixedWidth
    MyReader.FieldWidths = stdFormat
   

4. Loop through the rows, testing for format before reading.

   Dim currentRow As String()
   While Not MyReader.EndOfData
    Try
    Dim rowType = 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)
    Else
    ' Otherwise parse the fields normally
    MyReader.SetFieldWidths(stdFormat)
    End If
    currentRow = MyReader.ReadFields
    For Each newString In currentRow
    Console.Write(newString & "|")
    Next
    Console.WriteLine()
   

5. 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

The following is the complete example that 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 FileIO.TextFieldParser("..\..\testfile.txt")
 MyReader.TextFieldType = FileIO.FieldType.FixedWidth
 MyReader.FieldWidths = stdFormat
 Dim currentRow As String()
 While Not MyReader.EndOfData
 Try
 Dim rowType = 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)
 Else
 ' Otherwise parse the fields normally
 MyReader.SetFieldWidths(stdFormat)
 End If
 currentRow = MyReader.ReadFields
 For Each newString In currentRow
 Console.Write(newString & "|")
 Next
 Console.WriteLine()
 Catch ex As FileIO.MalformedLineException
 MsgBox("Line " & ex.Message & " is invalid. Skipping")
 End Try
 End While
End Using
Console.ReadLine()

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).

See also

• Microsoft.VisualBasic.FileIO.TextFieldParser
• PeekChars
• MalformedLineException
• WriteAllText
• EndOfData
• TextFieldType
• How to: Read From Comma-Delimited Text Files
• How to: Read From Fixed-width Text Files
• Parsing Text Files with the TextFieldParser Object