BuildingRequest event handler for OutlookServicesClient to insert user's email address into the Users segment

example.cs

// When using the v2 or beta endpoints of the Outlook API, the OutlookServicesClient class sends PATCH requests
// to a URL that contains the user's unique identifier in <guid>@<guid> format. This fails when authenticating
// with an app-only token.

// To work around this, you can implement an event handler for the BuildingRequest event on the client's .Context
// property, and replace the old user spec with the user's email.

// Sample function that calls an update on an event
public async Task<ActionResult> UpdateEvent(AppState passedAppState) 
{
  OutlookServicesClient client = new OutlookServicesClient(new Uri("https://outlook.office.com/api/v2.0"), async () =>
  {
    // Since we have it locally from the app state, just return it here.
    return passedAppState.AccessToken;
  });

  client.Context.BuildingRequest += new EventHandler<Microsoft.OData.Client.BuildingRequestEventArgs>(
  (sender, e) => FixAppOnlyUrl(sender, e, passedAppState.MailboxSmtpAddress));
  
  try 
  {
    var updateEvent = await client.Users[passedAppState.MailboxSmtpAddress].Events.GetById(passedAppState.EventId).ExecuteAsync();
    updateEvent.Subject = "modified: " + updateEvent.Subject;
    await updateEvent.UpdateAsync();
  }
  catch (Exception ex) 
  {
    ...
  }
}

// BuildingRequest event handler
private void FixAppOnlyUrl(object sender, BuildingRequestEventArgs e, string email)
{
  string escapedEmail = email.Replace("@", "%40");
  string oldUrl = e.RequestUri.ToString();
  // Only muck with the URL if it has the /Users() segment and
  // it doesn't already contain the user's email address
  if (oldUrl.ToLower().Contains("/users(") && !oldUrl.ToLower().Contains(escapedEmail.ToLower()))
  {
    string newUrl = Regex.Replace(oldUrl, "[Uu]sers\\('.+?'\\)", "Users('" + escapedEmail + "')");
    e.RequestUri = new Uri(newUrl);
  }
}