Question

What I currently have looks a bit like this:

if(userLikesBananas)
{
    return from fruit in basket
           select new Fruit
           {
               AteBanana = Bowl.Any(b => b.OwnedBy == user && b.Contains(fruit) && fruit.Type == FruitType.Banana),
               ...
               ...
               //lots of properties
               ...
           }
}
else
{
    return from fruit in basket
           select new Fruit
           {
               AteBanana = Bowl.Any(b => b.Contains(fruit)),
               ...
               ...
               //lots of properties
               ...
           }
}

Admittedly the example makes absolutely no sense, but the principle is that I want to change the conditions of a properties selection based on arbitrary criteria. Right now the select statements are repeated.

Now the time has come that I need to add anoter dependent criteria. I don’t want to have 4 different cases where the property conditions are slightly different.

What I want to do, is something like this:

Func<Fruit, bool> fruitFunc = f => false;

if(userLikesBananas)
{
    fruitFunc = f => Bowl.Any(b => b.OwnedBy == user && b.Contains(f) && f.Type == FruitType.Banana);
}
else
{
    fruitFunc = f => Bowl.Any(b => b.Contains(f));
}

return from fruit in basket
       select new Fruit
       {
           AteBanana = fruitFunc(fruit)
           ...
           ...
           //lots of properties
           ...
       };

The trouble is that is the expression cannot be converted to sql, as it contains a dynamic invoke. I have tried wrapping the Func in an Expression, but the same problem seems to arise.

So the question is, how can I avoid the copy and paste?

Answer 1

…my english is not good, but I’ll try to explain how to easily solve this problem 🙂

Dynamic Linq is bad for type control – It’s easy to use, but you can’t browse the resulting object (x.Name, x.Surname, etc.)

There is (a litle noob-like) trick to solve this problem (I’m using it and its working fain):

1. Create enum with attributes of your object you’re selecting from.

   public enum MyAtrs{ID, FirstName, Surname}

2. create Dictionary<MyAtrs,bool> (and fill it) for conditions (set true if you want to get this property)

   public Dictionary Dic = new Dictionary();
   Dic.Add(MyAtrs.ID,true);
   Dic.Add(MyAtrs.Firstname,false);
   Dic.Add(MyAtrs.Surname,true);

3. Built your query:

   var query = DBContext.MyDBTable.Where(predicate).Select(e=>new {
   ID = Dic[MyAtrs.ID] ? e.dbID:0,
   Firstname = Dic[MyAtrs.Firstname] ? e.dbFirstname:null,
   Surname = Dic[MyAtrs.Surname] ? e.dbSurname:null,
   });

In this case, there will be all 3 columns in the SQL Select statement, but it’s just a few bytes (who matters…). SQL Server gives you back all 3 columns but (in this case) Firstname will be empty (something like {ID=123, Firstname=, Surname=”Jobs”}).
It’s not brilliant but it’s easy way how to built “dynamic” select expression without loosing type control 🙂