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Back to Standard Query Operator Index


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Introduction

The Except operator produces the set difference between two sequences. It will only return elements in the first sequence that don't appear in the second. You can optionally provide your own equality comparison function.

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Method Signatures

// 1 - Returns only elements that appear in one of the sequences.
public static IEnumerable<TSource> Except<TSource>(
    this IEnumerable<TSource> first, 
    IEnumerable<TSource> second)
 
// New in January CTP
// 2 - Returns only elements that appear in one of the sequences.
// A custom function is used to determine equality.
public static IEnumerable<TSource> Except<TSource>(
    this IEnumerable<TSource> first, 
    IEnumerable<TSource> second, 
    IEqualityComparer<TSource> comparer)

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Exceptions

Throws an ArgumentNullException if first or second is null

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Pseudo-code

Overload 1
If first is null, throw an ArgumentNullException.
If second is null, throw an ArgumentNullException.
Add all the distinct second elements to a list (distinct element list)
Iterate the first sequence.

Overload 2
If first is null, throw an ArgumentNullException.
If second is null, throw an ArgumentNullException.
Add all the distinct second elements to a list (distinct element list)
Iterate the first sequence.

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Loop Count

The entire second sequence will be looped.
The first sequence follows: 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

This sample shows that the except operator will only return those elements in the first sequence that are not in the second sequence.

public static void ExceptTest() {
    int[] first  = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
    int[] second = new int[] {             4, 5, 6,         10 };
 
    var q = first.Except(second);
 
    foreach (var num in q)
        Console.WriteLine(num);
 
    Console.ReadLine();
}

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