Maintain subsequence order in OrderBy

Question

Suppose I have a class with an integer Day property and an IEnumerable of objects where the days are 2, 3, 4, 1, 3, 3 and 5 (in that order

Answer 1

Is there a way to guarantee that the order of the subsequence where

Assuming you have class like this:

class A
{
    public string Name { get; set; }
    public int Day { get; set; }

    // other properties
}

and the sequence:

{ "A", 2 },
{ "B", 3 },
{ "C", 4 },
{ "D", 1 },
{ "E", 3 },
{ "F", 3 },
{ "G", 5 },

If you mean, will this:

sequence.Where(item => item.Day == 3)

produce the sequence, where items will be ordered like this: B, E, F, then the answer is "no, you nave no guarantee".

If your sequence is a List, than ordering will be preserved (indeed, it will be preserved with LINQ to Objects, not only with lists).

If you sequence is IQueryable, then the ordering may depend from the LINQ provider implementation, underlying data source and current expression tree, already contained in IQueryable. So, in this case you should force ordering with OrderBy/OrderByDescending.