public sealed class LayoutView 

//Reference the active layout view.

//Confirm if the current, active view is a layout view.
//If it is, do something.
LayoutView activeLayoutView = LayoutView.Active;
if (activeLayoutView != null)
{
  // do something
}

//Get element's selection count.

//Count the number of selected elements on the active layout view
LayoutView activeLayoutView = LayoutView.Active;
if (activeLayoutView != null)
{
  var selectedElements = activeLayoutView.GetSelectedElements();
  ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show($@"Selected elements: {selectedElements.Count}");
}

//Set the active layout view's selection to include 2 rectangle elements.

//Reference the active view 
LayoutView activeLayoutView = LayoutView.Active;
if (activeLayoutView != null)
{

  //Perform on the worker thread
  QueuedTask.Run(() =>
  {

    //Reference the layout
    Layout lyt = activeLayoutView.Layout;

    //Reference the two rectangle elements
    Element rec = lyt.FindElement("Rectangle");
    Element rec2 = lyt.FindElement("Rectangle 2");

    //Construct a list and add the elements
    List<Element> elmList = new List<Element>
    {
      rec,
      rec2
    };

    //Set the selection
    activeLayoutView.SelectElements(elmList);
  });
}