Linq-element-operators

LINQの要素演算子

DefaultIfEmptyを除き、残りの8つの標準クエリ要素演算子はすべて、コレクションから単一の要素を返します。

Operator	Description	C# Query Expression Syntax	VB Query Expression Syntax
ElementAt	Returns an element present within a specific index in a collection	Not Applicable	Not Applicable
ElementAtOrDefault	Same as ElementAt except of the fact that it also returns a default value in case the specific index is out of range	Not Applicable	Not Applicable
First	Retrieves the first element within a collection or the first element satisfying a specific condition	Not Applicable	Not Applicable
FirstOrDefault	Same as First except the fact that it also returns a default value in case there is no existence of such elements	Not Applicable	Not Applicable
Last	Retrieves the last element present in a collection or the last element satisfying a specific condition	Not Applicable	Not Applicable
LastOrDefault	Same as Last except the fact that it also returns a default value in case there is no existence of any such element	Not Applicable	Not Applicable
Single	Returns the lone element of a collection or the lone element that satisfy a certain condition	Not Applicable	Not Applicable
SingleOrDefault	Same as Single except that it also returns a default value if there is no existence of any such lone element	Not Applicable	Not Applicable
DefaultIfEmpty	Returns a default value if the collection or list is empty or null	Not Applicable	Not Applicable

ElementAtの例-Enumerable.ElementAtメソッド

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Operators {
   class Program {
      static void Main(string[] args) {

         string[] names = { "Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow",
                       "Hedlund, Magnus", "Ito, Shu" };
         Random random = new Random(DateTime.Now.Millisecond);

         string name = names.ElementAt(random.Next(0, names.Length));

         Console.WriteLine("The name chosen at random is '{0}'.", name);
         Console.ReadLine();
      }
   }
}

VB

Module Module1

   Sub Main()

      Dim names() As String = _{"Hartono, Tommy", "Adams, Terry", "Andersen, Henriette Thaulow",
                         "Hedlund, Magnus", "Ito, Shu"}

      Dim random As Random = New Random(DateTime.Now.Millisecond)

      Dim name As String = names.ElementAt(random.Next(0, names.Length))

      MsgBox("The name chosen at random is " & name)

   End Sub

End Module

上記のC＃またはVBのコードをコンパイルして実行すると、次の結果が生成されます-

The name chosen at random is Ito, Shu

注-ここでは、上記の出力は動的に変化し、名前はランダムに選択されます。

Firstの例-Enumerable.Firstメソッド

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Operators {
   class Program {
      static void Main(string[] args) {

         int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54, 83, 23, 87, 435, 67, 12, 19 };

         int first = numbers.First();

         Console.WriteLine(first);
         Console.ReadLine();
      }
   }
}

VB

Module Module1

   Sub Main()

      Dim numbers() As Integer = _{9, 34, 65, 92, 87, 435, 3, 54, 83, 23, 87, 435, 67, 12, 19}

      Dim first As Integer = numbers.First()

      MsgBox(first)

   End Sub

End Module

上記のC＃またはVBのコードをコンパイルして実行すると、次の結果が生成されます-

9

Lastの例-Enumerable.Lastメソッド

C#

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Operators {
   class Program {
      static void Main(string[] args) {

         int[] numbers = { 9, 34, 65, 92, 87, 435, 3, 54, 83, 23, 87, 435, 67, 12, 19 };

         int last = numbers.Last();

         Console.WriteLine(last);
         Console.ReadLine();
      }
   }
}

VB

Module Module1

   Sub Main()

      Dim numbers() As Integer = _{9, 34, 65, 92, 87, 435, 3, 54, 83, 23, 87, 435, 67, 12, 19};

      Dim last As Integer = numbers.Last()

      MsgBox(last)

   End Sub

End Module

上記のC＃またはVBのコードをコンパイルして実行すると、次の結果が生成されます-

19