To retrieve the ContentType of a ListItem from the ListItemCollection using the GetItems method, there are two approaches that work.

1. If you are not specifying ViewFields in your CAML query then in your Load method you need to Include the ContentType. e.g.

clientContext.Load(listItems, items => items.Include(item => item.ContentType));

1. If you CAML query includes ViewFields then two modifications are required. The ViewFields needs to include the ContentTypeId field and the Load method needs to include the ContentType. e.g.

<ViewFields><FieldRef Name='ContentTypeId' /></ViewFields>

clientContext.Load(listItems, items => items.Include(item => item.ContentType));