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FirstOrDefault Operator

Modified: 2007/01/13 02:58 by t_magennis - Categorized as: LINQ to Objects
{TOC}
Namespace:System.Linq
Assembly:System.Core.dll
Extends:IEnumerable<T>

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Introduction

The FirstOrDefault operator returns the first element of a sequence, or a default value if no element is found.

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Method Signatures

// 1 - Return the first element of a sequence or default(TSource); if empty.
public static TSource FirstOrDefault<TSource>(
    this IEnumerable<TSource> source)
 
// 2 - Return the first element that passes a predicate function in the sequence.
// Return default(TSource) if the sequence is empty.
public static TSource FirstOrDefault<TSource>(
    this IEnumerable<TSource> source, 
    Func<TSource, bool> predicate)



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Exceptions

Throws an ArgumentNullException if source or predicate is null.


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Pseudo-code

Overload 1
If source is null, throw an ArgumentNullException.
If the source sequence implements IList<T>, then
If the source sequence is empty return default(TSource);.
Return source[0]. (performance optomization)
Else Iterate the source sequence.
If there is no first element, return default(TSource);.
Return the first (current element) element.

Overload 2
If source is null, throw an ArgumentNullException.
Iterate the source sequence.
If predicate(current element) returns true, return the current element.
Return default(TSource); if no elements pass the predicate function.


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Loop Count

Overload 1
< 1. If the collection is of type IList, then the first element is accessed by index position which is different depending on the collection implementation. Otherwise, an iterator returns the first element.


Overload 2
< 1. The source sequence is iterated, and the first matching element is returned.

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Code Samples

TODO:Needs code sample.

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