ascending/descending in LINQ - can one change the order via parameter?
C#LinqC# Problem Overview
I have a method which is given the parameter "bool sortAscending". Now I want to use LINQ to create sorted list depending on this parameter. I got then this:
var ascendingQuery = from data in dataList
orderby data.Property ascending
select data;
var descendingQuery = from data in dataList
orderby data.Property descending
select data;
As you can see, both queries differ only in "ascending" resp. "descending". I'd like to merge both queries, but I don't know how. Does anyone have the answer?
C# Solutions
Solution 1 - C#
You can easily create your own extension method on IEnumerable or IQueryable:
public static IOrderedEnumerable<TSource> OrderByWithDirection<TSource,TKey>
(this IEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
bool descending)
{
return descending ? source.OrderByDescending(keySelector)
: source.OrderBy(keySelector);
}
public static IOrderedQueryable<TSource> OrderByWithDirection<TSource,TKey>
(this IQueryable<TSource> source,
Expression<Func<TSource, TKey>> keySelector,
bool descending)
{
return descending ? source.OrderByDescending(keySelector)
: source.OrderBy(keySelector);
}
Yes, you lose the ability to use a query expression here - but frankly I don't think you're actually benefiting from a query expression anyway in this case. Query expressions are great for complex things, but if you're only doing a single operation it's simpler to just put that one operation:
var query = dataList.OrderByWithDirection(x => x.Property, direction);
Solution 2 - C#
In terms of how this is implemented, this changes the method - from OrderBy/ThenBy to OrderByDescending/ThenByDescending. However, you can apply the sort separately to the main query...
var qry = from .... // or just dataList.AsEnumerable()/AsQueryable()
if(sortAscending) {
qry = qry.OrderBy(x=>x.Property);
} else {
qry = qry.OrderByDescending(x=>x.Property);
}
Any use? You can create the entire "order" dynamically, but it is more involved...
Another trick (mainly appropriate to LINQ-to-Objects) is to use a multiplier, of -1/1. This is only really useful for numeric data, but is a cheeky way of achieving the same outcome.
Solution 3 - C#
What about ordering desc by the desired property,
blah = blah.OrderByDescending(x => x.Property);
And then doing something like
if (!descending)
{
blah = blah.Reverse()
}
else
{
// Already sorted desc ;)
}
Is it Reverse() too slow?
Solution 4 - C#
In addition to the beautiful solution given by @Jon Skeet, I also needed ThenBy and ThenByDescending, so I am adding it based on his solution:
public static IOrderedEnumerable<TSource> ThenByWithDirection<TSource, TKey>(
this IOrderedEnumerable<TSource> source,
Func<TSource, TKey> keySelector,
bool descending)
{
return descending ?
source.ThenByDescending(keySelector) :
source.ThenBy(keySelector);
}