public sealed class FolderConnectionProjectItem : ArcGIS.Desktop.Internal.Catalog.ProjectItemInfoCrawl, ArcGIS.Desktop.Core.IMetadata, ArcGIS.Desktop.Core.IProjectItem, ArcGIS.Desktop.Core.ISearchableItem, System.ComponentModel.INotifyPropertyChanged  

/// Get all the folder connections in a project
IEnumerable<FolderConnectionProjectItem> projectFolders = Project.Current.GetItems<FolderConnectionProjectItem>();

/// Get all the items that can be accessed from a folder connection. The items immediately 
/// contained by a folder, that is, the folder's children, are returned including folders
/// and individual items that can be used in ArcGIS Pro. This method does not return all 
/// items contained by any sub-folder that can be accessed from the folder connection.
FolderConnectionProjectItem folderConnection = Project.Current.GetItems<FolderConnectionProjectItem>()
                                                    .FirstOrDefault((folder => folder.Name.Equals("Data")));
await QueuedTask.Run(() =>
{
  IEnumerable<Item> folderContents = folderConnection.GetItems();
});

/// Remove a folder connection from a project; the folder stored on the local disk 
/// or the network is not deleted
FolderConnectionProjectItem folderToRemove = Project.Current.GetItems<FolderConnectionProjectItem>().FirstOrDefault(folder => folder.Name.Equals("PlantSpecies"));
if (folderToRemove != null)
  Project.Current.RemoveItem(folderToRemove as IProjectItem);

//Gets all the folder connections in the current project
var projectFolders = Project.Current.GetItems<FolderConnectionProjectItem>();
foreach (var FolderItem in projectFolders)
{
  //Do Something with the Folder connection.
}

//Gets a specific folder connection in the current project
FolderConnectionProjectItem myProjectFolder = Project.Current.GetItems<FolderConnectionProjectItem>().FirstOrDefault(folderPI => folderPI.Name.Equals("myDataFolder"));

// Remove a folder connection from a project; the folder stored on the local disk or the network is not deleted
FolderConnectionProjectItem folderToRemove = Project.Current.GetItems<FolderConnectionProjectItem>().FirstOrDefault(myfolder => myfolder.Name.Equals("PlantSpecies"));
if (folderToRemove != null)
  Project.Current.RemoveItem(folderToRemove as IProjectItem);

List<Item> _mxd = new List<Item>();
//Gets all the folder connections in the current project
var allFoldersItem = Project.Current.GetItems<FolderConnectionProjectItem>();
if (allFoldersItem != null)
{
  //iterate through all the FolderConnectionProjectItems found
  foreach (var folderItem in allFoldersItem)
  {
    //Search for mxd files in that folder connection and add it to the List<T>
    //Note:ArcGIS Pro automatically creates and dynamically updates a searchable index as you build and work with projects. 
    //Items are indexed when they are added to a project.
    //The first time a folder or database is indexed, indexing may take a while if it contains a large number of items. 
    //While the index is being created, searches will not return any results.
    _mxd.AddRange(folderItem.GetItems());
  }
}

var contentItem = Project.Current.GetItems<FolderConnectionProjectItem>().First();
//var contentItem = ...
//Check if the MCT is required for Refresh()
if (contentItem.IsMainThreadRequired)
{
  //QueuedTask.Run must be used if item.IsMainThreadRequired
  //returns true
  QueuedTask.Run(() => contentItem.Refresh());
}
else
{
  //if item.IsMainThreadRequired returns false, any
  //thread can be used to invoke Refresh(), though
  //BackgroundTask is preferred.
  contentItem.Refresh();

  //Or, via BackgroundTask
  ArcGIS.Core.Threading.Tasks.BackgroundTask.Run(() =>
    contentItem.Refresh(), ArcGIS.Core.Threading.Tasks.BackgroundProgressor.None);
}

//GetItems searches project content
var map = Project.Current.GetItems<MapProjectItem>().FirstOrDefault(m => m.Name == "Map1");
var layout = Project.Current.GetItems<LayoutProjectItem>().FirstOrDefault(m => m.Name == "Layout1");
var folders = Project.Current.GetItems<FolderConnectionProjectItem>();
var style = Project.Current.GetItems<StyleProjectItem>().FirstOrDefault(s => s.Name == "ArcGIS 3D");

//Find item uses a catalog path. The path can be to a file or dataset
var fcPath = @"C:\Pro\CommunitySampleData\Interacting with Maps\Interacting with Maps.gdb\Crimes";
var pdfPath = @"C:\Temp\Layout1.pdf";
var imgPath = @"C:\Temp\AddinDesktop16.png";

var fc = Project.Current.FindItem(fcPath);
var pdf = Project.Current.FindItem(pdfPath);
var img = Project.Current.FindItem(imgPath);