This is a follow-on from my last post on Modifying your EWS Managed API code to use Hybrid Modern Authentication against OnPrem Mailboxes . If instead of the EWS Managed API you are using EWS Proxy Code (generated from the EWS WSDL) and you want to migrate it to using Modern Authentication for Office365 and/or Hybrid here's a method you can use using the MSAL Authentication library.
Unlike the EWS Managed API the WSDL generated proxy classes and specifically the ExchangeServiceBinding class doesn't have any provision to use Token Credentials. One way of implementing this in .NET is to take advantage of Polymorphism and create a new class that is derived from the ExchangeServiceBinding class and then override the method GetWebResponse from this class (which is actually derived from the SoapHttpClientProtocol class which contains the actual method we are going to override https://docs.microsoft.com/en-us/dotnet/api/system.web.services.protocols.soaphttpclientprotocol.getwebrequest?view=netframework-4.8 )
At the same time we can also add the X-AnchorMailbox header into the request which is also recommended for any Exchange Online requests you make. And because this method is called before every EWS Request we can place our Token Refresh code in there. In this example I'm using which uses the MSAL all you need to include is code that fetches the token from the TokenCache, this will trigger a Token Refresh if need or ultimately throw to Interaction if the Refresh Token isn't available. So here is a basic C# Console App that can do Hybrid/Modern Auth discover using the MSAL library. If you want the project files you can download them from here