如何在ComboBox项中添加数值并在TextBox中显示计算出的值？

Question

如何在ComboBox的每个项目上添加一个数值并将其显示到VB.NET中的TextBox？

我尝试使用此代码：

Dim value As Integer

Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load 
    Me.combobox1.Items.Add("Single")
    Me.combobox1.Items.Add("Double")
End sub

Private Sub Roomtype_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)
    If combobox1.SelectedIndex = 0 Then
        value = Val(1000)
    ElseIf combobox1.SelectedIndex = 1 Then
        value = Val(2000)
End sub

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    txtTotal.Text = Val(value) * Val(textbox1.Text)
End sub

例如，我选择项目“ single”，然后将其值乘以2，我希望txtTotal.text中的输出为2000，但没有输出。