Does C# Distinct() method keep original ordering of sequence intact?

Question

I want to remove duplicates from list, without changing order of unique elements in the list.

Jon Skeet & others have suggested to use following

Answer 1

This highly depends on your linq-provider. On Linq2Objects you can stay on the internal source-code for Distinct, which makes one assume the original order is preserved.

However for other providers that resolve to some kind of SQL for example, that isn´t neccessarily the case, as an ORDER BY-statement usually comes after any aggregation (such as Distinct). So if your code is this:

myArray.OrderBy(x => anothercol).GroupBy(x => y.mycol);

this is translated to something similar to the following in SQL:

SELECT * FROM mytable GROUP BY mycol ORDER BY anothercol;

This obviously first groups your data and sorts it afterwards. Now you´re stuck on the DBMS own logic of how to execute that. On some DBMS this isn´t even allowed. Imagine the following data:

mycol anothercol
1     2
1     1
1     3
2     1
2     3

when executing myArr.OrderBy(x => x.anothercol).GroupBy(x => x.mycol) we assume the following result:

mycol anothercol
1     1
2     1

But the DBMS may aggregate the anothercol-column so, that allways the value of the first row is used, resulting in the following data:

mycol anothercol
1    2
2    1

which after ordering will result in this:

mycol anothercol
2    1
1    2

This is similar to the following:

SELECT mycol, First(anothercol) from mytable group by mycol order by anothercol;

which is the completely reverse order than what you expected.

You see the execution-plan may vary depending on what the underlying provider is. This is why there´s no guarantee about that in the docs.