Index out of range in listbox VB.NET - vb.net

Question

Consider the following code :
Private Sub DelButton_Click(sender As Object, e As EventArgs) Handles DelButton.Click
If OrderListBox.Text = "" = False Then
For i As Integer = OrderListBox.SelectedIndex To arr.Count - 1
arr.RemoveAt(i)
Next
OrderListBox.Items.Remove(OrderListBox.SelectedItem)
End If
calculate()
End Sub
The program crashes at arr.RemoveAt(i) and displays the following error:
Index was out of range. Must be non-negative and less than the size
of the collection.

Answer 1

First of all, please note that in VB.NET and C#, FOR loops are implemented differently!
In VB.NET, it works like this:
BEFORE the loop starts, you are determining start and end of the loop:
Start = OrderListBox.SelectedIndex
End = arr.Count-1
Then, the loop starts.
It is important to know, that in VB.NET, the end of the loop is NOT calculated again anymore. This is an important difference to C#. In C#, the end of the loop is calculated before each single loop.
And now, in the loop, you are DELETING records from the array.
Therefore, the count of records in the array is DECREASING.
However, your loop is going on, since you have calculated the count of records in the array before the loop started.
Therefore, you are going beyond the range of the array.
You could rewrite your code as follows:
Dim i as Integer
i = OrderListBox.SelectedIndex
while i < arr.Count
arr.RemoveAt(i)
Next
This article covers details about the for loop in VB.NET, especially the section "Technical Implementation": https://msdn.microsoft.com/en-us/library/5z06z1kb.aspx

Answer 2

This error will be thrown when you try to remove an item at an index greater than the size of the collection. i.e when i is greater than arr.Count - 1.
You should make sure that OrderListBox.SelectedIndex is not greater than arr.Count - 1. Because if it does, you remove an item that, well, does not exists.
This code is actually displayed in the MSDN docs. As stated, you should do something like this:
Private Sub RemoveTopItems()
' Determine if the currently selected item in the ListBox
' is the item displayed at the top in the ListBox.
If listBox1.TopIndex <> listBox1.SelectedIndex Then
' Make the currently selected item the top item in the ListBox.
listBox1.TopIndex = listBox1.SelectedIndex
End If
' Remove all items before the top item in the ListBox.
Dim x As Integer
For x = listBox1.SelectedIndex - 1 To 0 Step -1
listBox1.Items.RemoveAt(x)
Next x
' Clear all selections in the ListBox.
listBox1.ClearSelected()
End Sub 'RemoveTopItems

Answer 3

ListBox.SelectedIndex will return a value of negative one (-1) is returned if no item is selected.
You didn't check it in your code.
Change your code to this:
If OrderListBox.SelectedIndex >= 0 And OrderListBox.Text = "" = False Then
EDIT
Your code is this:
For i As Integer = OrderListBox.SelectedIndex To arr.Count - 1
arr.RemoveAt(i)
Next
Let's say your OrderListBox contains 3 items : [A, B, C] and the SelectedIndex is 0
Then your code will:
Remove (0) ===> [B, C]
Remove (1) ===> [B]
Remove (2) ===> Exception!
You need to reverse the loop
For i As Integer = arr.Count - 1 To OrderListBox.SelectedIndex Step -1
arr.RemoveAt(i)
Next