Ef core: Sequence contains no element when doing MaxAsync

Actually there is quite elegant (and more performant compared to the suggested in the other answer because it’s executing just a single database query) way by utilizing the fact that aggregate methods like Min, Max throw Sequence contains no element exception only when used with non nullable overloads, but nullable overloads simply return null instead.

So all you need is to promote the non nullable property type to the corresponding nullable type. For instance, if the OrderIndex type is int, the only change to your query could be

.MaxAsync(x => (int?)x.OrderIndex);

Note that this will also change the type of the receiving variable highestOrderIndex to int?. You can check for null and react accordingly, or you can simply combine the aggregate function call with ?? operator and provide some default value, for instance

var highestOrderIndex = (await _context.ExampleDbSet
    .Where(x => x.ForeignId == foreignId)
    .MaxAsync(x => (int?)x.OrderIndex)) ?? -1; // or whatever "magic" number works for you