CODEWITHSUNDEEP
vb.netindexoutofrangeexception
MoJoMan
MoJoMan

Reputation: 15

VB.NET Index out of Range exception related to text file

I have some code I have used many times over which has always worked great for me. The latest use, however, throws an exception under certain circumstances that I cannot seem to resolve. Here it is:

I read from a text file to an array, use it as a binding source for some of my controls (it autofills 3 controls based on the selection of a single control). I created a Student class with 4 properties (Name, ID, DOB and DOE). Here is the code I use:

Private Sub autoFill()
        Dim rost As String = "Roster.txt"
        Dim lines As List(Of String) = File.ReadAllLines(rost).ToList
        Dim list As List(Of Student) = New List(Of Student)
        For i As Integer = 0 To lines.Count - 1
            Dim data As String() = lines(i).Split(":")
            list.Add(New Student() With {
                     .StudentName = data(0),
                     .StudentID = data(1),
                     .StudentDOB = data(2),
                     .StudentDOE = data(3)
                                        })
        Next
        StudentBindingSource.DataSource = list


    End Sub

Now here is the problem. In the "For" loop when I set i to 0 to lines.count -1 it throws this error:

VB>NET EXCEPTION

However...If I change i to 1 instead of 0 it works OR if I take away data(2) and data(3) it works with i = 0. I would prefer to use 0 so that I can have a blank line in the combobox or "--choose--", etc. The only thing I have thought that might be useful is that my first row in the text file has nothing to split. Here is the line format of the text file:

Student Name         ID#        DOB          DOE      <-----This header row is NOT in the text file
Last Name, First Name : 0000000 : 01/01/2021 : 01/01/2021

I'm going to assume I'm missing something really simple here. Any guidance would be greatly appreciated! Thank you.

Upvotes: 0

Views: 1014

Answers (3)

Joel Coehoorn
Joel Coehoorn

Reputation: 415600

Before we get to the actual problem, let's re-work some things.

A better way to structure code, especially when working with data loading, is to have a method that accepts an input and returns a result. Additionally, calling ToList() or ToArray() is a very expensive operation for performance. Very often you can improve performance dramatically by working with a lower-level IEnumerable for as long as possible.

With those principles in mind, consider this code:

Private Function ReadStudentData(fileName As String) As IEnumerable(Of Student)
    Dim lines As IEnumerable(Of String) = File.ReadLines(fileName)

    Return lines.
        Select(Function(line) line.Split(":")).
        Select(Function(data)
            Return New Student() With {
                .StudentName = data(0),
                .StudentID = data(1),
                .StudentDOB = data(2),
                .StudentDOE = data(3)
            }
        End Function)
End Function

Private Sub autoFill()
    StudentBindingSource.DataSource = ReadStudentData("Roster.txt")
End Sub

Now on to the actual issue. The problem was not from looping through the list variable. The problem is the data array. At some point you have a line that doesn't have enough elements. This is common, for example, as the last line in a file.

There are many ways to address this. In some cases, the exception is already the appropriate result, because if you have bad data you really don't want to continue. In other cases you want to log the bad records, perhaps to a report you can easily review later. Or maybe you just want to ignore the error, or pre-filter for rows with the right number of columns. Here is an example of the last option:

Private Function ReadStudentData(fileName As String) As IEnumerable(Of Student)
    Return File.ReadLines(fileName).
        Select(Function(line) line.Split(":")).
        Where(Function(data) data.Length = 4).
        Select(Function(data) 
            Return New Student() With {
                .StudentName = data(0),
                .StudentID = data(1),
                .StudentDOB = data(2),
                .StudentDOE = data(3)
            }
        End Function)
End Function

Private Sub autoFill()
    StudentBindingSource.DataSource = ReadStudentData("Roster.txt")
End Sub

Upvotes: 2

Zeeshan Arif
Zeeshan Arif

Reputation: 499

try this code:

Dim list As List(Of Student) = New List(Of Student)(100)

basically initialize the student list with a capacity. This is the capacity of the list, not the count/length.

Upvotes: 0

dbasnett
dbasnett

Reputation: 11773

The problem is that you didn't check 'data' to have enough elements to create the 'Student'. A simple check should fix it.

Private Sub autoFill()
    Dim rost As String = "Roster.txt"
    Dim lines As List(Of String) = File.ReadAllLines(rost).ToList
    Dim list As List(Of Student) = New List(Of Student)
    For i As Integer = 0 To lines.Count - 1
        Dim data As String() = lines(i).Split(":"c)
        'Check data
        If data.Length >= 4 Then '
            list.Add(New Student() With {
                     .StudentName = data(0),
                     .StudentID = data(1),
                     .StudentDOB = data(2),
                     .StudentDOE = data(3)
                                        })
        End If
    Next
    StudentBindingSource.DataSource = list
End Sub

Upvotes: 0

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