How to start a thread with Dispatcher - wpf

Question

I am writing a code for Folder Browser Application on WPF, I wrote a code to get the folders using expend button, and wrote a function to calculate the size of the folders, now it looks like this Example of what am I have right now
This is the main part of the code:
This is my item_Expanded method:
void item_Expanded(object sender, RoutedEventArgs e)
{
var item = (TreeViewItem)sender;
if (this.HasDummy(item))
{
this.Cursor = Cursors.Wait;
this.RemoveDummy(item);
this.ExploreDirectories(item);
this.ExploreFiles(item);
this.Cursor = Cursors.Arrow;
}
}
it calls ExploreDirectories method:
private void ExploreDirectories(TreeViewItem item)
{
var directoryInfo = (DirectoryInfo)item.Tag;
foreach (var directory in directoryInfo.GetDirectories())
{
var isHidden = (directory.Attributes & FileAttributes.Hidden) == FileAttributes.Hidden;
var isSystem = (directory.Attributes & FileAttributes.System) == FileAttributes.System;
if (!isHidden && !isSystem)
{
item.Items.Add(this.GetItem(directory));
}
}
}
it calls GetItem method:
private TreeViewItem GetItem(DirectoryInfo directory)
{
var item = new TreeViewItem
{
Header = $"Name: {directory.Name} \t Size: {ConvertSize(GetDirectorySize(directory))}",
DataContext = directory,
Tag = directory
};
this.AddDummy(item);
item.Expanded += new RoutedEventHandler(item_Expanded);
return item;
}
and finaly it calls GetDirectorySize method:
static long GetDirectorySize(DirectoryInfo root, bool recursive = true)
{
FileInfo[] files = null;
DirectoryInfo[] subDirs = null;
var startDirectorySize = default(long);
{
files = root.GetFiles("*.*");
}
if (files != null)
{
foreach (var fileInfo in files)
Interlocked.Add(ref startDirectorySize, fileInfo.Length);
}
if (recursive)
{
subDirs = root.GetDirectories();
Parallel.ForEach(subDirs, (subDirectory) =>
Interlocked.Add(ref startDirectorySize, GetDirectorySize(subDirectory,
recursive)));
}
return startDirectorySize;
}
How should I rewrite my code to use a Dispatcher, so it stops freezing while calculating the size or expanding the folders and files? Should I rewrite my GetDirectorySize method, and change the Parallel.ForEach(), and Interlock.Add("recursive call here")?