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by Jiří {x2} Činčura

Disappearing order by clauses when composing query

7 Oct 2010 LINQ

If you’re composing queries in LINQ on various places, utilizing the delayed execution, you might be surprised, that some of your dynamically added OrderBys are not in final query.

Imagine we have a simple table with a and b columns (and primary key). If you write following query with ordering adding (it can be in different method etc.) ...

IQueryable<OrderingTest> tmp1 = context.OrderingTest;
tmp1 = tmp1.OrderBy(x => x.a);
tmp1 = tmp1.OrderBy(x => x.b);
//Console.WriteLine((tmp1 as ObjectQuery).ToTraceString());

... the result will contain sorting based only on b column. That’s because the last OrderBy took the precedence.

To make it work as expected you have to write it like this.

IQueryable<OrderingTest> tmp2 = context.OrderingTest;
tmp2 = tmp2.OrderBy(x => x.a);
if (tmp2 is IOrderedQueryable<OrderingTest>)
	tmp2 = (tmp2 as IOrderedQueryable<OrderingTest>).ThenBy(x => x.b);
//Console.WriteLine((tmp2 as ObjectQuery).ToTraceString());

The secondary (and further ordering) needs to be done via ThenBy method, which is available on IOrderedQueryable. That’s the reason for casting.

Sure it’s always safer to do this in one place directly, but sometimes the query is and has to be build on various places. Then think if a call to OrderBy could or couldn’t be at some place before.

You can always do some “dummy sort”, like _ => 0, initially and then use only ThenBy, but I personally don’t like playing with the intelligence of optimizer. It may have bad impact on performance.