Written February 26, 2014

LINQ Sequential Selects for Conditional Joins

I just finished a block of code that, while not ground breaking, is well worth recording. The problem setup is:

List<Foo> and List<Bar> conditionally joined to produce List<T>

What data we join on, however, is conditional on a property x of Foo being in List<Bar>. Let’s get a bit more concrete with our example since I hate completely abstract examples.

I have a List<AppetizerItems>, my wife has a List<DinnerItems>.
So lets put together our List<possibleMenu> with a few caveats. We would like to only go shopping at one store, be able to set the oven to one temperature and if the appetizer has has a wine, it would be nice if the dinner and appetizer shared the same color wine (i.e. red, white, rose, etc.).

var menu = appetizerItems 
    .Where( appetizerItem => 
            dinnerItems => 
                dinnerItems.Store == appetizerItem.Store 
                && dinnerItems.OvenTemperature == 
    .Select( appetizerItem => new 
        dinnerItem = ( 
            dinnerItems.Any(di => di.HasWine == appetizerItem.HasWine 
            ? dinerItems.First(di => 
                di.Store == appetizerItem.Store 
                && di.OvenTemperature == appetizerItem.OvenTemperature 
                && di.GrapeColor == appetizerItem.GrapeColor 
            :  dinnerItems.First(di => 
                di.Store == appetizerItem.Store 
                && di.OvenTemperature == appetizerItem.OvenTemperature 
    .Select(possibleMenu =>  new 

As you can see, we have managed to do a conditional join via anonymous object composition and sequential selects.