尝试使用 SelectedItem 这样的属性

Try using the SelectedItem Property like this

Private Sub ComboBox1_SelectedIndexChanged(sender As System.Object, e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
    If ComboBox1.SelectedItem.ToString = "PAKISTAN" Then
        ComboBox2.Visible = True
    End If
End Sub

由于您要检查多个国家/地区，您可以使用 选择案例 这样的声明

Since you are wanting to check for multiple country's you can use a Select Case Statement like this

Private Sub ComboBox1_SelectedIndexChanged(sender As System.Object, e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged

    Select Case ComboBox1.SelectedItem.ToString
        Case "PAKISTAN"
            ComboBox2.Visible = True
            ComboBox3.Visible = False
        Case "INDIA"
            ComboBox3.Visible = True
            ComboBox2.Visible = False
        Case Else
            ComboBox2.Visible = False
            ComboBox3.Visible = False
    End Select
End Sub