ArcGIS Pro 2.6 API Reference Guide
SetDefinition Method (Bookmark)
Example 

ArcGIS.Desktop.Mapping Namespace > Bookmark Class : SetDefinition Method
The definition used to update the bookmark.
Updates the bookmark using a CIM definition of a bookmark. This method must be called on the MCT. Use QueuedTask.Run.
Syntax
public void SetDefinition( 
   CIMBookmark bookmark
)
Public Sub SetDefinition( _
   ByVal bookmark As CIMBookmark _
) 

Parameters

bookmark
The definition used to update the bookmark.
Exceptions
ExceptionDescription
This method or property must be called within the lambda passed to QueuedTask.Run.
Example
Update the definition of a bookmark using a new extent.
public Task UpdateBookmarkExtentAsync(Bookmark bookmark, ArcGIS.Core.Geometry.Envelope envelope)
{
  return QueuedTask.Run(() =>
  {
    //Get the bookmark's definition
    var bookmarkDef = bookmark.GetDefinition();

    //Modify the bookmark's location
    bookmarkDef.Location = envelope;

    //Clear the camera as it is no longer valid.
    bookmarkDef.Camera = null;

    //Set the bookmark definition
    bookmark.SetDefinition(bookmarkDef);
  });
}
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

Bookmark Class
Bookmark Members