Visual Basic - system.nullReferenceException - vb.net

Question

So I'm still a bit of a newbie when it comes to programming, hence why I'm using visual basic. I'm getting this exception raised repeatedly, but the variables that vb is saying have unassigned values have been given values in my code. Can anyone point out where I'm going wrong with this?
EDIT: just a few more details: the file exists, I can read from it using just the ReadLine method, but I need to split the fields so I can compare the scores and get the highest 2 scores
Private Sub Form1_Load(sender As System.Object, e As System.EventArgs) Handles MyBase.Load
Dim srdFile As System.IO.StreamReader
Dim strLine As String
Dim strField(1) As String
Dim strName() As String
Dim strScore() As String
Dim i = 0
srdFile = New System.IO.StreamReader("HighScores.dat")
rtbOut.AppendText("HighScores:" & vbNewLine & vbNewLine)
Do Until srdFile.Peek() = -1
strLine = srdFile.ReadLine()
strField = strLine.Split(",")
strName(i) = strField(0)
strScore(i) = strField(1)
rtbOut.AppendText(strName(i) & ", " & strScore(i) & vbNewLine)
i = i + 1
Loop
End Sub

Answer 1

Following two arrays are never initialized: strName and strScore
I don't know the logic, but one way would be to use a List(Of String) instead which does not need to get the correct size in the first place and can be resized. I would also use the Using-statement to dispose the stream properly:
Using srdFile As New System.IO.StreamReader("HighScores.dat")
Dim strLine As String
Dim strField(1) As String
Dim strName As New List(Of String)
Dim strScore As New List(Of String)
Dim i = 0
rtbOut.AppendText("HighScores:" & vbNewLine & vbNewLine)
Do Until srdFile.Peek() = -1
strLine = srdFile.ReadLine()
strField = strLine.Split(","c)
strName.Add(strField(0))
strScore.Add(strField(1))
rtbOut.AppendText(strName(i) & ", " & strScore(i) & vbNewLine)
i += 1
Loop
End Using
Side-note: i recommend to set Option Strict to On by default.
By the way, here is a completely different approach doing the same but with LINQ:
Dim lines = From line In IO.File.ReadLines("HighScores.dat")
Where Not String.IsNullOrWhiteSpace(line)
Let fields = line.Split(","c)
Let name = fields.First()
Let score = fields.Last()
Select String.Format("{0}, {1}", name, score)
rtbOut.Text = String.Join(Environment.NewLine, lines)
I find this more readable.

Answer 2

Before you use an array, you need to assign a fixed array size in the computer memory locations. You can do this by initialising an array with the number of array elements. In your code, you have not allocated any memory to strName() and strScore() before using them, hence the code will throw an exception.
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Dim srdFile As System.IO.StreamReader
Dim strLine As String
Dim strField(1) As String
Dim strName(10) As String ''fixed size array (Using List(Of T) is a better option)
Dim strScore(10) As String ''fixed size array (Using List(Of T) is a better option)
Dim i = 0
srdFile = New System.IO.StreamReader("HighScores.dat")
rtbOut.AppendText("HighScores:" & vbNewLine & vbNewLine)
Do Until srdFile.Peek() = -1
strLine = srdFile.ReadLine()
strField = strLine.Split(",")
strName(i) = strField(0)
strScore(i) = strField(1)
rtbOut.AppendText(strName(i) & ", " & strScore(i) & vbNewLine)
i = i + 1
Loop
End Sub
You can also create a dynamic array. Please follow Resizing an array at runtime in VB.NET on Stackoverflow about dynamic array.