Not able to understand the functionality of ToLookup linq extension method

Question

I am trying to figure out the functionality of ToLookup method here in the code.It's using grouping somehow to return a list. Can somebody just tell me how this is working or just guide me in the right direction:

.ToLookup(
  c => new 
  {
      c.CoverageType.Code, 
      c.CoverageType.Mnemonic, 
      c.CoverageType.Description, 
      c.CoverageType.CoverageLevel, 
      c.CoverageType.CoveragePosition
  })
.Select(
  t => new CoverageOption
  {
    CoverageType =
      new CoverageType
      {
          Code = t.Key.Code, 
          Mnemonic = t.Key.Mnemonic, 
          Description = t.Key.Description, 
          CoverageLevel = t.Key.CoverageLevel, 
          CoveragePosition = t.Key.CoveragePosition
      },
    Limits =
      (from Coverage c in t select c.Limit).GroupBy(l => l.Code)
        .Select(gr => gr.First()), 
    Deductibles =
      (from Coverage c in t select c.Deductible).GroupBy(d => d.Code)
        .Select(gr => gr.First())
})
.OrderBy(t => t.CoverageType.CoveragePosition);

Which bit don't you understand? You use `ToLookup` to create a one to many dictionary (the lambda function is the key selector function). You can see this later in the select where you are using `t.Key...` in the select — RobH, Oct 30 '12 at 10:24

score 1 · Answer 1 · answered Oct 30 '12 at 10:51

Source items (which I assume has type Coverage) are grouped by this key:

new 
{
  c.CoverageType.Code, 
  c.CoverageType.Mnemonic, 
  c.CoverageType.Description, 
  c.CoverageType.CoverageLevel, 
  c.CoverageType.CoveragePosition
}

For each distinct key in the input sequence a CoverageOption is created. Note that a single key may be associated with multiple Coverage items and for each key and one or more associated Coverage items the following properties of CoverageOption is computed:

The CoverageType property is computed from the key.
The Limits property is computed by taking all the Limit values of the associated Coverage items and for each distinct Coverage.Limit.Code pick the first Limit instance. So this is a sequence of Limit objects each having a distinct Code.
The Deductibles property is computed by taking all the Deductible values of the associated Coverage items and for each distinct Coverage.Deductible.Code pick the first Deductible instance. So this is a sequence of Deductible objects each having a distinct Code.

Wow, this answer is so awesome. I am going through it, i am sure this would clear the clouds!! Thanks a lot man! — Charu, Oct 30 '12 at 11:00
so the key for this ToLookup was a Tuple initially which i changed to a anonymous type as in the code above. Can you comment on this, if what i did was right? I removed tuples because i had to access the keys using items1, 2 etc which made the code unreadable. — Charu, Oct 30 '12 at 11:03
@Charu: Instead of using an anonymous type as the key you should be able to use `CoverageType`. However, to do that you need to override `CoverageType.Equals` and `CoverageType.GetHashCode` to ensure that different instances with same property values are considered equal. — Martin Liversage, Oct 30 '12 at 11:15

score 0 · Answer 2 · edited May 23 '17 at 10:25

0

As documentation states:

The ToLookup(IEnumerable, Func) method returns a Lookup, a one-to-many dictionary that maps keys to collections of values. A Lookup differs from a Dictionary, which performs a one-to-one mapping of keys to single values.

so it is similar to GroupBy but there are some differences that are described here.

edited May 23 '17 at 10:25

Community

1
1

answered Oct 30 '12 at 10:27

Rafal

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Not able to understand the functionality of ToLookup linq extension method

2 Answers2