ArcGIS Pro 2.7 API Reference Guide
CreateLegend Method (IElementFactory)
Example 

ArcGIS.Desktop.Layouts Namespace > IElementFactory Interface : CreateLegend Method
Layout or GroupElement
Envelope
MapFrame
Creates a legend element on a layout from an envelope geometry. This method must be called on the MCT. Use QueuedTask.Run.
Syntax

Parameters

elementContainer
Layout or GroupElement
envelope
Envelope
mapFrame
MapFrame

Return Value

Returns a Legend.
Exceptions
ExceptionDescription
This method or property must be called within the lambda passed to QueuedTask.Run.
Remarks
If the element container is a Layout then the element gets added to the root level of the layout TOC at the top most position. If the element container is a GroupElement then it gets added to the group at the topmost position.
Example
//Create a legend for an associated map frame.

//Construct on the worker thread
await QueuedTask.Run(() =>
{
  //Build 2D envelope geometry
  Coordinate2D leg_ll = new Coordinate2D(6, 2.5);
  Coordinate2D leg_ur = new Coordinate2D(8, 4.5);
  Envelope leg_env = EnvelopeBuilder.CreateEnvelope(leg_ll, leg_ur);

  //Reference MF, create legend and add to layout
  MapFrame mapFrame = layout.FindElement("New Map Frame") as MapFrame;
  if (mapFrame == null)
  {
    ArcGIS.Desktop.Framework.Dialogs.MessageBox.Show("Map frame not found", "WARNING");
    return;
  }
  Legend legendElm = LayoutElementFactory.Instance.CreateLegend(layout, leg_env, mapFrame);
  legendElm.SetName("New Legend");
});
Requirements

Target Platforms: Windows 10, Windows 8.1

See Also

Reference

IElementFactory Interface
IElementFactory Members