Get the ComboBox selected item in ComboBox_SelectionChanged method

I am following the MVVM pattern with C# and WPF. In my .xaml.cs file, I have a combobox_SelectionChanged method where I need to access the selected item of the combobox.

     var cb=sender as ComboBox;
     Type t=cd.getType();

I tried to get the type and then the object bound to SelectedItem property. That was not scuccessful.

Then I tried the following. ComboBoxItem cbi = (ComboBoxItem)cb.SelectedItem;

      string text = cbi.Content.ToString();

even that was not successful. Can someone suggest me a way to do that ?