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{TOC}

Back to Standard Query Operator Index


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Introduction

The OrderBy operator orders a sequence according to one or more keys in descending order.

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Method Signatures

// 1 - Order the sequence in reverse alphabetical order.
public static IOrderedSequence<TSource> OrderByDescending<TSource, TKey>(
    this IEnumerable<TSource> source, 
    Func<TSource, TKey> keySelector)
 
// 2 - Order the sequence in reverse order. Use a custom comparer function.
public static IOrderedSequence<TSource> OrderByDescending<TSource, TKey>(
    this IEnumerable<TSource> source, 
    Func<TSource, TKey> keySelector, 
    IComparer<TKey> comparer)

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Exception

Throws an ArgumentNullException if source or keySelector is null.

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Pseudo-code

Overload 1
If source is null, throw an ArgumentNullException.
If keySelector is null, throw an ArgumentNullException.
Iterate the source sequence Iterate the buffer sequence.
Sort the buffer collection of keys in reverse order using Quicksort algorithm.
Iterate the buffer collection, now sorted.

Overload 2 - custom comparer.
If source is null, throw an ArgumentNullException.
If keySelector is null, throw an ArgumentNullException.
If comparer is null, use Comparer<TKey>.Default; instead.
Iterate the source sequence.
Iterate the buffer sequence.
Sort the buffer collection of keys in reverse order using Quicksort algorithm.
Iterate the buffer collection, now sorted.

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Loop Count

2 - 2.5. A copy of the source elements is made, the keys evaluated, then the sequence is sorted by a Quicksort algorithm. This operator implements the standard deferred execution iterator pattern. This means, no looping will occur until the result is iterated over..

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Code Samples

TODO: Needs code sample.