ArcGIS Pro 2.8 API Reference Guide
SelectAllElements Method (LayoutView)
Example 

ArcGIS.Desktop.Layouts Namespace > LayoutView Class : SelectAllElements Method
Select all layout elements in a layout view. This method must be called on the MCT. Use QueuedTask.Run.
Syntax
public void SelectAllElements()
Public Sub SelectAllElements() 
Exceptions
ExceptionDescription
This method or property must be called within the lambda passed to QueuedTask.Run.
Example
//Select all element on a layout.

layoutView.SelectAllElements();
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

LayoutView Class
LayoutView Members