GroupBy LINQ中的两个属性之一

Question

I know its easy to group objects by two properties but I was wondering is it possible to Group by either of two properties? Probably below example will make it more clear:-

Suppose I have this table:-

CategoryID        Fruit
-------------------------
1                 Apple
1                 Grapes
2                 Tablet
2                 Laptop
5                 Coke
6                 Coke

Now, i want to group this data either if CategoryIds are same or Fruits are same.

Sample Output:-

Group 1:-
1          Apple
1          Grapes

Group 2:-
2          Tablet
2          Laptop

Group 3:-
5          Coke
6          Coke

Answer 1

To add some details to your requirements it seems that you want to do a normal group by on CategoryID . Then, if any groups only contain a single element you want these to be grouped by Fruit . The key of this group will have to contain both a CategoryID and a Fruit . For groups 1 and 2 in your sample data the Fruit property will be the default value for Fruit . For group 3 the CategoryID will be the default value for CategoryID .

You can perform this computation by doing two nested groupings and then unwrapping the inner grouping again:

var groupedItems = items
  .GroupBy(item => item.CategoryID)
  .GroupBy(
    inner => inner.Count() > 1
      ? new { CategoryID = inner.Key, Fruit = default(String) }
      : new { CategoryID = default(Int32), inner.First().Fruit }
  )
  .Select(
    outer => outer.Count() == 1
      ? new { outer.Key, Items = outer.First().ToList() }
      : new { outer.Key, Items = outer.Select(inner => inner.First()).ToList() }
  );

Given this input

var items = new[] {
  new { CategoryID = 1, Fruit = "Apple" },
  new { CategoryID = 1, Fruit = "Coke" },
  new { CategoryID = 2, Fruit = "Tablet" },
  new { CategoryID = 2, Fruit = "Coke" },
  new { CategoryID = 5, Fruit = "Coke" },
  new { CategoryID = 6, Fruit = "Coke" }
};

you get this output

Key             |Items
----------------+-----------------
CategoryId Fruit|CategoryID Fruit
----------------+-----------------
1          null |1          Apple
                |1          Coke
----------------+-----------------
2          null |2          Tablet
                |2          Coke
----------------+-----------------
0          Coke |5          Coke
                |6          Coke
----------------+-----------------