vb.net - Visual Basic の "Property chars is read only" とは何ですか? どうすれば修正できますか?

Question

私の目標はこれを行うことです: ファイルから配列にデータを読み取り、リスト ボックスをロードし、各学生の平均を計算し、文字の成績を決定し、クラスの平均を計算します。

私の宣言された変数（グローバル）：

Dim project, midterm, hw1, hw2, final, count, average As Integer

Dim letterGrade, lastName, firstName As String
Dim classAverage As Double

---

私のコード:

Dim array1 As String() = New String(6) {}

    Dim streamreader1 As StreamReader = File.OpenText("grades.csv")
    count = 0
    classAverage = 0
    Do While (streamreader1.Peek() <> -1)
        array1 = Strings.Split(streamreader1.ReadLine(), ",", -1)
        firstName(count) = array1(0)
        lastName(count) = array1(1)
        lstNames.Items.Add(lastName(count) & ", " & firstName(count))
        hw1(count) = Convert.ToString(array1(2))
        hw2(count) = Convert.ToString(array1(3))
        project(count) = Convert.ToString(array1(4))
        midterm(count) = Convert.ToString(array1(5))
        final(count) = Convert.ToString(array1(6))
        average(count) = Math.Round(Math.Truncate((((((hw1(count) + hw2(count)) + (2 * project(count))) + (4 * (midterm(count) + final(count)))) / 12) + 0.5)))
        classAverage = (classAverage + ((((hw1(count) + hw2(count)) + (2 * project(count))) + (4 * (midterm(count) + final(count)))) / 12))
        Dim num1 As Integer = average(count)
        If (num1 >= 93) Then
            letterGrade(count) = "A"
        ElseIf (num1 >= 90) Then
            letterGrade(count) = "A-"
        ElseIf (num1 >= 87) Then
            letterGrade(count) = "B+"
        ElseIf (num1 >= 83) Then
            letterGrade(count) = "B"
        ElseIf (num1 >= 80) Then
            letterGrade(count) = "B-"
        ElseIf (num1 >= 77) Then
            letterGrade(count) = "C+"
        ElseIf (num1 >= 73) Then
            letterGrade(count) = "C"
        ElseIf (num1 >= 70) Then
            letterGrade(count) = "C-"
        ElseIf (num1 >= 67) Then
            letterGrade(count) = "D+"
        ElseIf (num1 >= 63) Then
            letterGrade(count) = "D"
        ElseIf (num1 >= 60) Then
            letterGrade(count) = "D"
        Else
            letterGrade(count) = "E"
        End If
        count = (count + 1)

    Loop
    classAverage = (classAverage / count)
    lstNames.SelectedIndex = 0
    PopulateLabels(0)
    streamreader1.Close()