ArcGIS Pro 2.6 API Reference Guide
ShowDialog Method (OpenItemDialog)
Example 

ArcGIS.Desktop.Catalog Namespace > OpenItemDialog Class : ShowDialog Method
Displays the Browse dialog box to open or use selcted items
Syntax
public override Nullable<bool> ShowDialog()
Public Overrides NotOverridable Function ShowDialog() As Nullable(Of Boolean)

Return Value

A boolean indicating if the Browse dialog was dismissed by pressing OK or Cancel
Remarks
You can either configure the Browse dialog box properties first then display it with the ShowDialog method, or display the dialog box and then set its properties.
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