成绩计算器：值对于小数而言太大或太小

Question

我正在尝试编写一个等级计算器，用于计算学生总数中合格和不及格的学生。唯一的问题是，如果我将这些字段留空，则会产生一个错误：“对于小数，值太大或太小。”

公共类frmMain

Private dblTotal As Double
Private dblPassing As Double
Private dblFailing As Double

Private decFailing As Decimal
Private decPassing As Decimal

Private Sub btnCalc_Click(sender As Object, e As EventArgs) Handles btnCalc.Click

    'Parse text fields as Integers or Decimals
    Double.TryParse(txtPassing.Text, dblPassing)
    Double.TryParse(txtFailing.Text, dblFailing)
    Double.TryParse(lblTotalDisplay.Text, dblTotal)

    Decimal.TryParse(lblPassingDisplay.Text, decPassing)
    Decimal.TryParse(lblFailingDisplay.Text, decFailing)

    'Adds both passing and failing students
    dblTotal = (dblPassing + dblFailing)

    'Calculate passing students
    decPassing = (dblPassing / dblTotal)

    'Calculate failing students
    decFailing = (dblFailing / dblTotal)

    'Display total number of students
    lblTotalDisplay.Text = dblTotal.ToString("N0")

    'Display passing and failing percentages
    lblPassingDisplay.Text = decPassing.ToString("P2")
    lblFailingDisplay.Text = decFailing.ToString("P2")

End Sub

Private Sub btnClear_Click(sender As Object, e As EventArgs) Handles btnClear.Click

    txtPassing.Text = String.Empty
    txtFailing.Text = String.Empty

    lblFailingDisplay.Text = String.Empty
    lblPassingDisplay.Text = String.Empty
    lblTotalDisplay.Text = String.Empty

End Sub

Private Sub btnExit_Click(sender As Object, e As EventArgs) Handles btnExit.Click
    Me.Close()
End Sub

结束班级