Issue when Declaring a ComboBox Object using C# code

I'm making a program for window phone using silverlight.

I have a small problem and it is about creating a combo box.

I can create it using xaml, but I am in a situation where I should declare it using c# code.

The problem is after creating a new instance of the combo box and adding the items to it, the combo box doesn't appear!!

The code:

        TextBlock tb = new TextBlock();
        tb.Text = "Select your arrival status";

        tb.Margin = new Thickness(5.0);
        tb.FontSize = 20;
        tb.Foreground = new SolidColorBrush(Colors.White);

        ComboBox cb = new ComboBox();
        ObservableCollection<string> testList = new ObservableCollection<string>();
        testList.Add("Hi");
        testList.Add("Hi1");
        testList.Add("Hi2");
        cb.DataContext = testList;
        cb.Height = 50;
        cb.Width = 200;
        cb.Foreground = new SolidColorBrush(Colors.White);

        panel.Children.Add(tb);
        panel.Children.Add(cb);

As you can see from the code, I have declared a TextBlock and a Combobox objects. The TextBox object appears whereas the the Combobox doesn't.

Any help will be appreciated.

Copyright Notice：Content Author:「boring91」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/15432788/issue-when-declaring-a-combobox-object-using-c-sharp-code