Highlight All Files within a Folder in Standard Windows Open File Dialog

The Open File dialog accepts multiple file names as a space-separated list of quoted items:

"C:\MyFile1.txt" "C:\MyFile2.txt"

OpenFile() sets the dialog's file name input as is, so you can try the following:

// JavaScript/JScript
dlgOpen.OpenFile("\"C:\\MyFile1.txt\" \"C:\\MyFile2.txt\"");

Alternatively, you can use the Ctrl+A shortcut to select all files in the Open dialog:

var strFolder = "C:\\MyFolder\\Subfolder\\";
var dlgOpen = ...;

// Navigate to the folder
var fileInput = dlgOpen.Window("ComboBoxEx32").FindChild("WndClass", "Edit", 2);
fileInput.SetText(strFolder);
fileInput.Keys("[Enter]");

// Select all files & open
dlgOpen.FindChild("WndClass", "SHELLDLL_DefView", 5).Keys("^a");
dlgOpen.Window("Button", "&Open").Click();

This was tested on Windows 7. Other Windows versions can have different object names for the Open dialog elements, so adjust accordingly.