Enumerable.LastOrDefault メソッド

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source);

public static TSource LastOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source);

public static TSource LastOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource>? source);

static member LastOrDefault : seq<'Source> -> 'Source

<Extension()>
Public Function LastOrDefault(Of TSource) (source As IEnumerable(Of TSource)) As TSource

型パラメーター

パラメーター

戻り値

TSource

ソース シーケンスが空の場合は default (TSource)。それ以外の場合は IEnumerable<T> の最後の要素。default(TSource) if the source sequence is empty; otherwise, the last element in the IEnumerable<T>.

例外

例

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

string[] fruits = { };
string last = fruits.LastOrDefault();
Console.WriteLine(
    String.IsNullOrEmpty(last) ? "<string is null or empty>" : last);

/*
 This code produces the following output:

 <string is null or empty>
*/

' Create an empty array.
Dim fruits() As String = {}

' Get the last item in the array, or a
' default value if there are no items.
Dim last As String = fruits.LastOrDefault()

' Display the result.
Console.WriteLine(IIf(String.IsNullOrEmpty(last),
       "<string is Nothing or empty>",
       last))

' This code produces the following output:
'
' <string is Nothing or empty>

の値は、 default(TSource) コレクションに要素が含まれていない場合に使用する既定値ではない場合があります。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確認し、必要に応じて変更するのではなく、メソッドを使用して、 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できます。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、 Last<TSource>(IEnumerable<TSource>) を呼び出して最後の要素を取得します。Then, call Last<TSource>(IEnumerable<TSource>) to obtain the last element. 次のコード例では、月の日数のコレクションが空の場合に、両方の手法を使用して既定値1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 days of the month is empty. 整数の既定値は0であるため、月のどの日にも対応していません。そのため、既定値は1として指定する必要があります。Because the default value for an integer is 0, which does not correspond to any day of the month, the default value must be specified as 1 instead. 最初の結果変数は、クエリの実行が完了した後に、不要な既定値に対してチェックされます。The first result variable is checked for the unwanted default value after the query has finished executing. 2番目の結果変数は、を使用して既定値1を指定して取得され DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) ます。The second result variable is obtained by using DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) to specify a default value of 1.

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

// Setting the default value to 1 after the query.
int lastDay1 = daysOfMonth.LastOrDefault();
if (lastDay1 == 0)
{
    lastDay1 = 1;
}
Console.WriteLine("The value of the lastDay1 variable is {0}", lastDay1);

// Setting the default value to 1 by using DefaultIfEmpty() in the query.
int lastDay2 = daysOfMonth.DefaultIfEmpty(1).Last();
Console.WriteLine("The value of the lastDay2 variable is {0}", lastDay2);

/*
 This code produces the following output:

 The value of the lastDay1 variable is 1
 The value of the lastDay2 variable is 1
*/

Dim daysOfMonth As New List(Of Integer)(New Integer() {})

' Setting the default value to 1 after the query.
Dim lastDay1 As Integer = daysOfMonth.LastOrDefault()
If lastDay1 = 0 Then
    lastDay1 = 1
End If
Console.WriteLine($"The value of the lastDay1 variable is {lastDay1}")

' Setting the default value to 1 by using DefaultIfEmpty() in the query.
Dim lastDay2 As Integer = daysOfMonth.DefaultIfEmpty(1).Last()
Console.WriteLine($"The value of the lastDay2 variable is {lastDay2}")

' This code produces the following output:
'
' The value of the lastDay1 variable is 1
' The value of the lastDay2 variable is 1

注釈

参照型と null 許容型の既定値は null です。The default value for reference and nullable types is null.

メソッドには、 LastOrDefault 既定値を指定する方法は用意されていません。The LastOrDefault method does not provide a way to specify a default value. 以外の既定値を指定する場合は、「 default(TSource) DefaultIfEmpty<TSource>(IEnumerable<TSource>, TSource) 例」のセクションで説明されているように、メソッドを使用します。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.

public:
generic <typename TSource>
[System::Runtime::CompilerServices::Extension]
 static TSource LastOrDefault(System::Collections::Generic::IEnumerable<TSource> ^ source, Func<TSource, bool> ^ predicate);

public static TSource LastOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);

public static TSource? LastOrDefault<TSource> (this System.Collections.Generic.IEnumerable<TSource> source, Func<TSource,bool> predicate);

static member LastOrDefault : seq<'Source> * Func<'Source, bool> -> 'Source

<Extension()>
Public Function LastOrDefault(Of TSource) (source As IEnumerable(Of TSource), predicate As Func(Of TSource, Boolean)) As TSource

型パラメーター

パラメーター

戻り値

TSource

シーケンスが空の場合、または述語関数でテストに合格する要素がない場合は default (TSource)。それ以外の場合は、述語関数でテストに合格する最後の要素。default(TSource) if the sequence is empty or if no elements pass the test in the predicate function; otherwise, the last element that passes the test in the predicate function.

例外

例

次のコード例は、述語を渡してを使用する方法を示し LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) ています。The following code example demonstrates how to use LastOrDefault<TSource>(IEnumerable<TSource>, Func<TSource,Boolean>) by passing in a predicate. メソッドの2回目の呼び出しでは、条件を満たす要素がシーケンス内に存在しません。In the second call to the method, there is no element in the sequence that satisfies the condition.

double[] numbers = { 49.6, 52.3, 51.0, 49.4, 50.2, 48.3 };

double last50 = numbers.LastOrDefault(n => Math.Round(n) == 50.0);

Console.WriteLine("The last number that rounds to 50 is {0}.", last50);

double last40 = numbers.LastOrDefault(n => Math.Round(n) == 40.0);

Console.WriteLine(
    "The last number that rounds to 40 is {0}.",
    last40 == 0.0 ? "<DOES NOT EXIST>" : last40.ToString());

/*
 This code produces the following output:

 The last number that rounds to 50 is 50.2.
 The last number that rounds to 40 is <DOES NOT EXIST>.
*/

' Create an array of doubles.
Dim numbers() As Double = {49.6, 52.3, 51.0, 49.4, 50.2, 48.3}

' Get the last item whose value rounds to 50.0.
Dim number50 As Double =
numbers.LastOrDefault(Function(n) Math.Round(n) = 50.0)

Dim output As New System.Text.StringBuilder
output.AppendLine("The last number that rounds to 50 is " & number50)

' Get the last item whose value rounds to 40.0.
Dim number40 As Double =
numbers.LastOrDefault(Function(n) Math.Round(n) = 40.0)

Dim text As String = IIf(number40 = 0.0,
                     "[DOES NOT EXIST]",
                     number40.ToString())
output.AppendLine("The last number that rounds to 40 is " & text)

' Display the output.
Console.WriteLine(output.ToString)

' This code produces the following output:
'
' The last number that rounds to 50 is 50.2
' The last number that rounds to 40 is [DOES NOT EXIST]

注釈

参照型と null 許容型の既定値は null です。The default value for reference and nullable types is null.