public string GetSignOnUsername()

// A portal project path looks like this:
//@"https://<ServerName>.<Domain>.com/portal/sharing/rest/content/items/1a434faebbe7424d9982f57d00223baa";
//A local project path looks like this:
//@"C:\Users\<UserName>\Documents\ArcGIS\Projects\MyProject\MyProject.aprx";

//Check if the project can be opened
if (Project.CanOpen(projectPath, out docVer))
{
  //Open the project
  await Project.OpenAsync(projectPath);
}
else //The project cannot be opened
{
  //One possible reason: If the project is a portal project, the active portal must match the portal of the project
  //Check if this is a portal project
  bool isPortalProject = Uri.TryCreate(projectPath, UriKind.Absolute, out Uri uriResult)
       && (uriResult.Scheme == Uri.UriSchemeHttp || uriResult.Scheme == Uri.UriSchemeHttps);

  if (isPortalProject)
  {
    //Parse the project path to get the portal
    var uri = new Uri(projectPath);
    var portalUrlOfProjectToOpen = $"{uri.Scheme}://{uri.Host}/portal/";

    //Get the current active portal
    var activePortal = ArcGIS.Desktop.Core.ArcGISPortalManager.Current.GetActivePortal();
    //Compare to see if the active Portal is the same as the portal of the project
    bool isSamePortal = (activePortal != null && activePortal.PortalUri.ToString() == portalUrlOfProjectToOpen);
    if (!isSamePortal) //not the same. 
    {
      //Set new active portal to be the portal of the project
      //Find the portal to sign in with using its Uri...
      var projectPortal = ArcGISPortalManager.Current.GetPortal(new Uri(portalUrlOfProjectToOpen, UriKind.Absolute));
      await QueuedTask.Run(() => {
        if (!projectPortal.IsSignedOn())
        {
          //Calling "SignIn" will trigger the OAuth popup if your credentials are
          //not cached (eg from a previous sign in in the session)
          if (projectPortal.SignIn().success)
          {
            //Set this portal as my active portal
            ArcGISPortalManager.Current.SetActivePortal(projectPortal);
            return;
          }
        }
        //Set this portal as my active portal
        ArcGISPortalManager.Current.SetActivePortal(projectPortal);
      });
      //Now try opening the project again
      if (Project.CanOpen(projectPath, out docVer))
      {
        await Project.OpenAsync(projectPath);
      }
      else
      {
        System.Diagnostics.Debug.WriteLine("The project cannot be opened.");
      }
    }
    else //The portals are the same. So the problem could be something else - permissions, portal is down?
    {
      System.Diagnostics.Debug.WriteLine("The project cannot be opened.");
    }
  }
  else //Project is on disk and cannot be opened. 
  {
    System.Diagnostics.Debug.WriteLine("The project cannot be opened.");
  }
}

var projectPortal = ArcGISPortalManager.Current.GetPortal(new Uri(@"https://<serverName>.<domain>.com/portal/", UriKind.Absolute));
string owner = string.Empty;
await QueuedTask.Run(() => {
  //Get the signed on user name
  owner = projectPortal.GetSignOnUsername();
});
//Get the user content from the portal
var userContent = await projectPortal.GetUserContentAsync(owner);
//Get the first portal project item
var firstPortalProject = userContent.PortalItems.FirstOrDefault(pi => pi.PortalItemType == PortalItemType.ProProject);
var portalProjectUri = firstPortalProject.ItemUri.ToString();
//Check if project can be opened
string docVer = string.Empty;
if (Project.CanOpen(portalProjectUri, out docVer))
{
  await Project.OpenAsync(portalProjectUri);
}
//Note: If Project.CanOpen returns false, the project cannot be opened. One reason could be 
// the active portal is not the same as the portal of the project. Refer to the snippet: [Workflow to open an ArcGIS Pro project](ProSnippets-sharing#workflow-to-open-an-arcgis-pro-project)

var portal = ArcGISPortalManager.Current.GetActivePortal();
//Force login
if (!portal.IsSignedOn())
{
  portal.SignIn();
}
var user = portal.GetSignOnUsername();

var portal = ArcGISPortalManager.Current.GetActivePortal();
var owner = portal.GetSignOnUsername();
var userContent = await portal.GetUserContentAsync(owner);
//Get content for a specific folder (identified by its folder id)
//var userContent = await portal.GetUserContentAsync(owner, folderId);

//Get all the folders
foreach (var pf in userContent.PortalFolders)
{
  //Do something with the folders

}

//Get all the content items
foreach (var pi in userContent.PortalItems)
{
  //Do something with the portal items
}

var portal = ArcGISPortalManager.Current.GetPortal(portalUri);
var owner = portal.GetSignOnUsername();
var portalInfo = await portal.GetPortalInfoAsync();

//1. Get all web maps
var query1 = PortalQueryParameters.CreateForItemsOfType(PortalItemType.WebMap);

//2. Get all web maps and map services - include user, organization
// and "usa" in the title
var query2 = PortalQueryParameters.CreateForItemsOfTypes(new List<PortalItemType>() {
  PortalItemType.WebMap, PortalItemType.MapService}, owner, "", "title:usa");
query2.OrganizationId = portalInfo.OrganizationId;

//retrieve in batches of up to a 100 each time
query2.Limit = 100;

//Loop until done
var portalItems = new List<PortalItem>();
while (query2 != null)
{
  //run the search
  PortalQueryResultSet<PortalItem> results = await portal.SearchForContentAsync(query2);
  portalItems.AddRange(results.Results);
  query2 = results.NextQueryParameters;
}

//process results
foreach (var pi in portalItems)
{
  //Do something with the portal items
}