This question is not specific to graph, but if you are trying to cast a list of objects to a single object. You'll need to get the class that implements that interface (IChatMessageHostedContentsCollectionPage), create a new instance of it, and add your list to whatever property on the page contains the items
you can check similar question to related to how to cast objects - https://stackoverflow.com/questions/63939881/unable-to-cast-object-of-type-listmicrosoft-graph-chatmessagehostedcontent-to
Hope this helps
Thanks
, could you please try using mapper.Map<EventMessageDetails>(c.EventDetails).