ArcGIS Pro 2.6 API Reference Guide
MultiSelect Property
Example 

ArcGIS.Desktop.Catalog Namespace > OpenItemDialog Class : MultiSelect Property
Gets and sets if the Browse dialog box will support selecting many items
Syntax
public bool MultiSelect {get; set;}
Public Property MultiSelect As Boolean
Remarks
By default, you can only select a single item in the Browse dialog box.
Example
OpenItemDialog addToProjectDialog = new OpenItemDialog();
addToProjectDialog.Title = "Add To Project";
addToProjectDialog.InitialLocation = @"C:\Data\NewYork\Counties\Maps";
addToProjectDialog.MultiSelect = true;
addToProjectDialog.Filter = ItemFilters.composite_maps_import;

bool? ok = addToMapDialog.ShowDialog();

if (ok == true)
{
  IEnumerable<Item> selectedItems = addToMapDialog.Items;
  foreach (Item selectedItem in selectedItems)
    MapFactory.Instance.CreateMapFromItem(selectedItem);
}
Requirements

Target Platforms: Windows 10, Windows 8.1, Windows 7

See Also

Reference

OpenItemDialog Class
OpenItemDialog Members