How is ording a List of objects handled with Linq and or Sort?

Question

Sorry if this is a duplicate. I'm having a hard time finding an answer because I get so much bloat of people asking how to sort lists in general in my search results.

Say I have a property like this:

public List<object> Hello { get; set; }

Say each element in the list is of the same nullable primitive data type.

So every element is a int? or every element is a decimal? or every element is a DateTime?, etc

What exactly happens if I invoke a Sort or OrderBy on this list? Does it sort based on the underlying type of each element or does it order by the string value of the property?

Answer 1

Let's check http://ideone.com/kcXmHE :

System.ArgumentException: Value does not fall within the expected range.
  at System.String.CompareTo (System.Object value) [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.Comparer`1+DefaultComparer[System.Object].Compare (System.Object x, System.Object y) [0x00000] in <filename unknown>:0 
  at System.Linq.SortSequenceContext`2[System.Object,System.Object].Compare (Int32 first_index, Int32 second_index) [0x00000] in <filename unknown>:0 
  at System.Array.qsort[Int32] (System.Int32[] keys, Int32 low0, Int32 high0, IComparer`1 comparer) [0x00000] in <filename unknown>:0 
  at System.Array.SortImpl[Int32] (System.Int32[] keys, Int32 index, Int32 length, IComparer`1 comparer) [0x00000] in <filename unknown>:0 
  at System.Array.Sort[Int32] (System.Int32[] array, IComparer`1 comparer) [0x00000] in <filename unknown>:0 
  at System.Linq.QuickSort`1[System.Object].PerformSort () [0x00000] in <filename unknown>:0 
  at System.Linq.QuickSort`1+<Sort>c__Iterator0[System.Object].MoveNext () [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.List`1[System.Object]..ctor (IEnumerable`1 collection) [0x00000] in <filename unknown>:0 
  at System.Linq.Enumerable.ToList[Object] (IEnumerable`1 source) [0x00000] in <filename unknown>:0 
  at Test.Main () [0x00000] in <filename unknown>:0 

So, LINQ uses default comparer for element of list (for string value you can see System.String.CompareTo ) and tries to compare with another value (of object type). And, can't do it.

If you change order of elements in initial list ( http://ideone.com/foljli ), you will get another stack trace:

System.ArgumentException: Object must be of type Int32.
  at System.Int32.CompareTo (System.Object value) [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.Comparer`1+DefaultComparer[System.Object].Compare (System.Object x, System.Object y) [0x00000] in <filename unknown>:0 
  at System.Linq.SortSequenceContext`2[System.Object,System.Object].Compare (Int32 first_index, Int32 second_index) [0x00000] in <filename unknown>:0 
  at System.Array.qsort[Int32] (System.Int32[] keys, Int32 low0, Int32 high0, IComparer`1 comparer) [0x00000] in <filename unknown>:0 
  at System.Array.SortImpl[Int32] (System.Int32[] keys, Int32 index, Int32 length, IComparer`1 comparer) [0x00000] in <filename unknown>:0 
  at System.Array.Sort[Int32] (System.Int32[] array, IComparer`1 comparer) [0x00000] in <filename unknown>:0 
  at System.Linq.QuickSort`1[System.Object].PerformSort () [0x00000] in <filename unknown>:0 
  at System.Linq.QuickSort`1+<Sort>c__Iterator0[System.Object].MoveNext () [0x00000] in <filename unknown>:0 
  at System.Collections.Generic.List`1[System.Object]..ctor (IEnumerable`1 collection) [0x00000] in <filename unknown>:0 
  at System.Linq.Enumerable.ToList[Object] (IEnumerable`1 source) [0x00000] in <filename unknown>:0 
  at Test.Main () [0x00000] in <filename unknown>:0 

Now, LINQ takes int32 and tries to compare with object. And fail again. Because int32 type has no method CompareTo(object)