Wait! You have confused everything! This has nothing to do with VisualStudio. And either you use WPF or WindowsForms control. PresentationFramework.dll is WPF, the link is for the WindowsForms control.
To allow user to select all elements in wpf listbox look here:
http://stackoverflow.com/questions/1030276/wpf-listbox-and-select-all[
^]
If you want to select programatically all elements, you can use the
SelectedItems[
^] property, and add all elements from the bound source (don't forget to set
multiple selection mode[
^])