LINQ查询的结果是否始终保证正确的顺序？

Question

Question: Will the result of a LINQ query always be guaranteed to be in the correct order?

Example:

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 

var lowNums = 
  from n in numbers 
  where n < 5 
  select n; 

Now, when we walk through the entries of the query result, will it be in the same order as the input data numbers is ordered?

foreach (var x in lowNums) 
  { 
    Console.WriteLine(x); 
  } 

If someone can provide a note on the ordering in the documentation, this would be perfect.

Answer 1

• For LINQ to Objects : Yep.
• For Parallel LINQ : Nope.
• For LINQ to Expression Trees (EF, L2S, etc): Nope.

Answer 2

I think the order of elements retrieved by a LINQ is preserved, at least for LINQ to Object , for LINQ to SQL or Entity , it may depend on the order of the records in the table. For LINQ to Object , I'll try explaining why it preserves the order.

In fact when the LINQ query is executed, the IEnumerable source will call to GetEnumerator() to start looping with a while loop and get the next element using MoveNext() . This is how a foreach works on the IEnumerable source. We all know that a foreach will preserve the order of the elements in a list/collection. Digging more deeply into the MoveNext() , I think it just has some Position to save the current Index and MoveNext() just increase the Position and yield the corresponding element (at the new position). That's why it should preserve the order, all the code changing the original order is redundant or by explicitly calling to OrderBy or OrderByDescending .

If you think this

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 
foreach(var i in numbers)
  if(i < 5) Console.Write(i + "  ");

prints out 4 1 3 2 0 you should think this

int[] numbers = { 5, 4, 1, 3, 9, 8, 6, 7, 2, 0 }; 
IEnumerator ie = numbers.GetEnumerator();
while(ie.MoveNext()){
  if((int)ie.Current < 5) Console.Write(ie.Current + "  ");
}

also prints out 4 1 3 2 0 . Hence this LINQ query

var lowNums = from n in numbers 
              where n < 5 
              select n;
foreach (var i in lowNums) { 
   Console.Write(i + "  "); 
}

should also print out 4 1 3 2 0 .

Conclusion: The order of elements in LINQ depends on how MoveNext() of an IEnumerator obtained from an IEnumerable is implemented. However, it's for sure that the order of elements in LINQ result will be the same order a foreach loop works on the elements .