问VB中医院收费功能的几个问题

EN

Const DayRate As Decimal = 350
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
    'declare variables
    Dim PhysicalRehab As Decimal
    Dim SurgeryCharges As Decimal
    Dim LabCharges As Decimal
    Dim MedCharges As Decimal
    Dim StayCharges As Decimal
    Try
        'Copy the scores into the variables
        StayCharges = CDec(TextBoxLength.Text) * DayRate
        MedCharges = CDec(TextBoxMed.Text)
        SurgeryCharges = CDec(TextBoxSurg.Text)
        LabCharges = CDec(TextBoxLab.Text)
        PhysicalRehab = CDec(TextBoxPhy.Text)
    Catch
        MessageBox.Show("Please enter numeric values", "Error")
        Return
    End Try
    'Find out if box enteries are negative or not
    If Convert.ToDecimal(LabCharges) < 0 Then
        MessageBox.Show("No Negative Numbers", "Try Agian")
        Return
        TextBoxLab.SelectAll()
    ElseIf Convert.ToDecimal(MedCharges) < 0 Then
        MessageBox.Show("No Negative Numbers", "Try Agian")
        Return
        TextBoxMed.SelectAll()
    ElseIf Convert.ToDecimal(SurgeryCharges) < 0 Then
        MessageBox.Show("No Negative Numbers", "Try Agian")
        Return
        TextBoxSurg.SelectAll()
    ElseIf Convert.ToDecimal(PhysicalRehab) < 0 Then
        MessageBox.Show("No Negative Numbers", "Try Agian")
        Return
        TextBoxPhy.SelectAll()
    ElseIf Convert.ToDecimal(StayCharges) < 0 Then
        MessageBox.Show("No Negative Numbers", "Try Agian")
        Return
        TextBoxLength.SelectAll()
    End If
    'Sends results to the label
    LabelTotal.Text = CalctotalCharges.ToString("c")
End Sub
Function CalcMiscCharges() As Decimal
    Return CDec(TextBoxMed.Text) + CDec(TextBoxLab.Text) + CDec(TextBoxPhy.Text) + CDec(TextBoxSurg.Text)
End Function
Function CalcChargesOfStay() As Decimal
    Return CalcChargesOfStay = CDec(TextBoxLength.Text) * DayRate
End Function

Function CalctotalCharges() As Decimal
    Return CDec(CalctotalCharges = CDec(CalcChargesOfStay() + CalcMiscCharges()))
End Function

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

Private Sub ButtonClear_Click(sender As Object, e As EventArgs) Handles ButtonClear.Click
    TextBoxLab.Clear()
    TextBoxLength.Clear()
    TextBoxMed.Clear()
    TextBoxPhy.Clear()
    TextBoxSurg.Clear()
End Sub