ArcGIS Pro 2.6 API Reference Guide
SelectAllElements Method
Example 

ArcGIS.Desktop.Layouts Namespace > LayoutView Class : SelectAllElements Method
Select all layout elements in a layout view.
Syntax
public void SelectAllElements()
Public Sub SelectAllElements() 
Example
//Select all element on a layout.

layoutView.SelectAllElements();
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

LayoutView Class
LayoutView Members