Enumerable.FirstOrDefault Enumerable.FirstOrDefault Enumerable.FirstOrDefault Enumerable.FirstOrDefault Method

定義

シーケンスの最初の要素を返します。要素が見つからない場合は既定値を返します。Returns the first element of a sequence, or a default value if no element is found.

オーバーロード

FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

条件を満たす、シーケンスの最初の要素を返します。このような要素が見つからない場合は既定値を返します。Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.

FirstOrDefault<TSource>(IEnumerable<TSource>) FirstOrDefault<TSource>(IEnumerable<TSource>) FirstOrDefault<TSource>(IEnumerable<TSource>) FirstOrDefault<TSource>(IEnumerable<TSource>)

シーケンスの最初の要素を返します。シーケンスに要素が含まれていない場合は既定値を返します。Returns the first element of a sequence, or a default value if the sequence contains no elements.

FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)

条件を満たす、シーケンスの最初の要素を返します。このような要素が見つからない場合は既定値を返します。Returns the first element of the sequence that satisfies a condition or a default value if no such element is found.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);
public static TSource FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);
static member FirstOrDefault : seq<'Source> * Func<'Source, bool> -> 'Source
<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean)) As TSource

型パラメーター

TSource

source の要素の型。The type of the elements of source.

パラメーター

source
IEnumerable<TSource>

返される要素が含まれる IEnumerable<T>An IEnumerable<T> to return an element from.

predicate
Func<TSource,Boolean>

各要素が条件を満たしているかどうかをテストする関数。A function to test each element for a condition.

戻り値

TSource TSource TSource TSource

TSource が空の場合または source で指定されたテストに合格する要素がない場合は default(predicate)。それ以外の場合は、source で指定されたテストに合格する、predicate の最初の要素。default(TSource) if source is empty or if no element passes the test specified by predicate; otherwise, the first element in source that passes the test specified by predicate.

例外

source または predicatenull です。source or predicate is null.

次のコード例は、使用する方法を示しますFirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>)述語内で渡すことで。The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. メソッドの 2 番目の呼び出しで要素が存在しない条件を満たす配列にします。In the second call to the method, there is no element in the array that satisfies the condition.

string[] names = { "Hartono, Tommy", "Adams, Terry", 
                     "Andersen, Henriette Thaulow", 
                     "Hedlund, Magnus", "Ito, Shu" };

string firstLongName = names.FirstOrDefault(name => name.Length > 20);

Console.WriteLine("The first long name is '{0}'.", firstLongName);

string firstVeryLongName = names.FirstOrDefault(name => name.Length > 30);

Console.WriteLine(
    "There is {0} name longer than 30 characters.",
    string.IsNullOrEmpty(firstVeryLongName) ? "not a" : "a");

/*
 This code produces the following output:

 The first long name is 'Andersen, Henriette Thaulow'.
 There is not a name longer than 30 characters.
*/
' Create an array of strings.
Dim names() As String =
{"Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow", "Hedlund, Magnus", "Ito, Shu"}

' Select the first string in the array whose length is greater than 20.
Dim firstLongName As String =
names.FirstOrDefault(Function(name) name.Length > 20)

' Display the output.
MsgBox("The first long name is " & firstLongName)

' Select the first string in the array whose length is greater than 30,
' or a default value if there are no such strings in the array.
Dim firstVeryLongName As String =
names.FirstOrDefault(Function(name) name.Length > 30)

Dim text As String = IIf(String.IsNullOrEmpty(firstVeryLongName), "not a", "a")

MsgBox("There is " & text & " name longer than 30 characters.")

' This code produces the following output:
'
' The first long name is Andersen, Henriette Thaulow
'
' There is not a name longer than 30 characters.

注釈

参照および null 許容型の既定値はnullします。The default value for reference and nullable types is null.

FirstOrDefault<TSource>(IEnumerable<TSource>) FirstOrDefault<TSource>(IEnumerable<TSource>) FirstOrDefault<TSource>(IEnumerable<TSource>) FirstOrDefault<TSource>(IEnumerable<TSource>)

シーケンスの最初の要素を返します。シーケンスに要素が含まれていない場合は既定値を返します。Returns the first element of a sequence, or a default value if the sequence contains no elements.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource FirstOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source);
public static TSource FirstOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);
static member FirstOrDefault : seq<'Source> -> 'Source
<Extension()>
Public Function FirstOrDefault(Of TSource) (source As IEnumerable(Of TSource)) As TSource

型パラメーター

TSource

source の要素の型。The type of the elements of source.

パラメーター

source
IEnumerable<TSource>

最初の要素を返す IEnumerable<T>The IEnumerable<T> to return the first element of.

戻り値

TSource TSource TSource TSource

TSource が空の場合は default(source)。それ以外の場合は source の最初の要素。default(TSource) if source is empty; otherwise, the first element in source.

例外

次のコード例は、使用する方法を示しますFirstOrDefault<TSource>(IEnumerable<TSource>)で空の配列。The following code example demonstrates how to use FirstOrDefault<TSource>(IEnumerable<TSource>) on an empty array.

int[] numbers = { };
int first = numbers.FirstOrDefault();
Console.WriteLine(first);

/*
 This code produces the following output:

 0
*/
' Create an empty array.
Dim numbers() As Integer = {}

' Select the first element in the array, or a default value
' if there are not elements in the array.
Dim first As Integer = numbers.FirstOrDefault()

' Display the output.
MsgBox(first)

' This code produces the following output:
'
' 0

値もdefault(TSource)コレクションに要素が含まれていない場合は、使用する既定値はありません。Sometimes the value of default(TSource) is not the default value that you want to use if the collection contains no elements. 不要な既定値の結果をチェックし、必要に応じてを変更して、代わりに使用することができます、DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)コレクションが空の場合に使用する既定値を指定します。Instead of checking the result for the unwanted default value and then changing it if necessary, you can use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method to specify the default value that you want to use if the collection is empty. 次に呼び出すFirst<TSource>(IEnumerable<TSource>)最初の要素を取得します。Then, call First<TSource>(IEnumerable<TSource>) to obtain the first element. 次のコード例では、両方の手法を使用して、月を表す数値のコレクションが空の場合は、1 の既定値を取得します。The following code example uses both techniques to obtain a default value of 1 if a collection of numeric months is empty. 整数の既定値は 0 で、任意の月に対応していない、ため、既定値必要があります 1 として代わりに指定します。Because the default value for an integer is 0, which does not correspond to any month, the default value must be specified as 1 instead. クエリの実行が完了した後、不要な既定値の最初の結果変数がチェックされます。The first result variable is checked for the unwanted default value after the query has finished executing. 2 番目の結果の変数を使用して取得されますDefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)1 の既定値を指定します。The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.

List<int> months = new List<int> { };

// Setting the default value to 1 after the query.
int firstMonth1 = months.FirstOrDefault();
if (firstMonth1 == 0)
{
    firstMonth1 = 1;
}
Console.WriteLine("The value of the firstMonth1 variable is {0}", firstMonth1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int firstMonth2 = months.DefaultIfEmpty(1).First();
Console.WriteLine("The value of the firstMonth2 variable is {0}", firstMonth2);

/*
 This code produces the following output:

 The value of the firstMonth1 variable is 1
 The value of the firstMonth2 variable is 1
*/
Dim months As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim firstMonth1 As Integer = months.FirstOrDefault()
If firstMonth1 = 0 Then
    firstMonth1 = 1
End If
MsgBox(String.Format("The value of the firstMonth1 variable is {0}", firstMonth1))

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim firstMonth2 As Integer = months.DefaultIfEmpty(1).First()
MsgBox(String.Format("The value of the firstMonth2 variable is {0}", firstMonth2))

' This code produces the following output:
'
' The value of the firstMonth1 variable is 1
' The value of the firstMonth2 variable is 1

注釈

参照および null 許容型の既定値はnullします。The default value for reference and nullable types is null.

FirstOrDefaultメソッドが既定値を指定する方法を提供していません。The FirstOrDefault method does not provide a way to specify a default value. 以外の既定値を指定したい場合default(TSource)を使用して、DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource)メソッドの使用例」で説明します。If you want to specify a default value other than default(TSource), use the DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) method as described in the Example section.

適用対象