List<T>.FindLast(Predicate<T>) List<T>.FindLast(Predicate<T>) List<T>.FindLast(Predicate<T>) List<T>.FindLast(Predicate<T>) Method

Definition

Sucht nach einem Element, das die durch das angegebene Prädikat definierten Bedingungen erfüllt, und gibt das letzte Vorkommen im gesamten List<T> zurück.Searches for an element that matches the conditions defined by the specified predicate, and returns the last occurrence within the entire List<T>.

public:
 T FindLast(Predicate<T> ^ match);
public T FindLast (Predicate<T> match);
member this.FindLast : Predicate<'T> -> 'T
Public Function FindLast (match As Predicate(Of T)) As T

Parameter

match
Predicate<T> Predicate<T> Predicate<T> Predicate<T>

Der Predicate<T>-Delegat, der die Bedingungen für das Element definiert, nach dem gesucht werden soll.The Predicate<T> delegate that defines the conditions of the element to search for.

Gibt zurück

T T T T

Das letzte Element, das die durch das angegebene Prädikat definierten Bedingungen erfüllt, sofern vorhanden, andernfalls der Standardwert für den Typ T.The last element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type T.

Ausnahmen

Beispiele

Im folgenden Beispiel werden die Find-Methoden für List<T> die-Klasse veranschaulicht.The following example demonstrates the find methods for the List<T> class. Das Beispiel für die List<T> -Klasse book enthält die-Objekte Bookder-Klasse mithilfe der Daten aus der XML-Beispieldatei: Bücher (LINQ to XML).The example for the List<T> class contains book objects, of class Book, using the data from the Sample XML File: Books (LINQ to XML). Die FillList -Methode im Beispiel verwendet LINQ to XML , um die Werte aus der XML-Datei mit den book Eigenschafts Werten der-Objekte zu analysieren.The FillList method in the example uses LINQ to XML to parse the values from the XML to property values of the book objects.

In der folgenden Tabelle werden die Beispiele für die Find-Methoden beschrieben.The following table describes the examples provided for the find methods.

MethodeMethod BeispielExample
Find(Predicate<T>) Sucht ein Buch mit dem IDToFind Prädikat Delegaten über eine ID.Finds a book by an ID using the IDToFind predicate delegate.

C#Beispiel verwendet einen anonymen Delegaten.C# example uses an anonymous delegate.
FindAll(Predicate<T>) Suchen aller Bücher, deren Genre Eigenschaft "Computer" ist, mit FindComputer dem Prädikat Delegaten.Find all books that whose Genre property is "Computer" using the FindComputer predicate delegate.
FindLast(Predicate<T>) Sucht das letzte Buch in der Auflistung mit einem Veröffentlichungsdatum vor 2001 mit dem PubBefore2001 Prädikat Delegaten.Finds the last book in the collection that has a publish date before 2001, using the PubBefore2001 predicate delegate.

C#Beispiel verwendet einen anonymen Delegaten.C# example uses an anonymous delegate.
FindIndex(Predicate<T>) Sucht den Index des ersten Computer Buchs mit dem FindComputer Prädikat Delegaten.Finds the index of first computer book using the FindComputer predicate delegate.
FindLastIndex(Predicate<T>) Sucht den Index des letzten Computer Buchs mit dem FindComputer Prädikat Delegaten.Finds the index of the last computer book using the FindComputer predicate delegate.
FindIndex(Int32, Int32, Predicate<T>) Sucht den Index des ersten Computer Buchs in der zweiten Hälfte der Sammlung mit dem FindComputer Prädikat Delegaten.Finds the index of first computer book in the second half of the collection, using the FindComputer predicate delegate.
FindLastIndex(Int32, Int32, Predicate<T>) Sucht den Index des letzten Computer Buchs in der zweiten Hälfte der Sammlung mit dem FindComputer Prädikat Delegaten.Finds the index of last computer book in the second half of the collection, using the FindComputer predicate delegate.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Xml.Linq;

namespace Find
{
    class Program
    {
        private static string IDtoFind = "bk109";

        private static List<Book> Books = new List<Book>();
        public static void Main(string[] args)
        {
            FillList();
            
            // Find a book by its ID.
            Book result = Books.Find(
            delegate(Book bk)
            {
                return bk.ID == IDtoFind;
            }
            );
            if (result != null)
            {
                DisplayResult(result, "Find by ID: " + IDtoFind);   
            }
            else
            {
                Console.WriteLine("\nNot found: {0}", IDtoFind);
            }


            // Find last book in collection published before 2001.
            result = Books.FindLast(
            delegate(Book bk)
            {
                DateTime year2001 = new DateTime(2001,01,01);
                return bk.Publish_date < year2001;
            });
            if (result != null)
            {
                DisplayResult(result, "Last book in collection published before 2001:");
            }
            else
            {
                Console.WriteLine("\nNot found: {0}", IDtoFind);
            }


            // Find all computer books.
            List<Book> results = Books.FindAll(FindComputer);
            if (results.Count != 0)
            {
                DisplayResults(results, "All computer:");
            }
            else
            {
                Console.WriteLine("\nNo books found.");
            }

            // Find all books under $10.00.
            results = Books.FindAll(
            delegate(Book bk)
            {
                return bk.Price < 10.00;
            }
            );
            if (results.Count != 0)
            {
                DisplayResults(results, "Books under $10:");
            }
            else
            {
                Console.WriteLine("\nNo books found.");
            }
            

            // Find index values.
            Console.WriteLine();
            int ndx = Books.FindIndex(FindComputer);
            Console.WriteLine("Index of first computer book: {0}", ndx);
            ndx = Books.FindLastIndex(FindComputer);
            Console.WriteLine("Index of last computer book: {0}", ndx);

            int mid = Books.Count / 2;
            ndx = Books.FindIndex(mid, mid, FindComputer);
            Console.WriteLine("Index of first computer book in the second half of the collection: {0}", ndx);
            
            ndx = Books.FindLastIndex(Books.Count - 1, mid, FindComputer);
            Console.WriteLine("Index of last computer book in the second half of the collection: {0}", ndx);

        }





        // Populates the list with sample data.
        private static void FillList()
        {
            
            // Create XML elements from a source file.
            XElement xTree = XElement.Load(@"c:\temp\books.xml");

            // Create an enumerable collection of the elements.
            IEnumerable<XElement> elements = xTree.Elements();

            // Evaluate each element and set set values in the book object.
            foreach (XElement el in elements)
            {
                Book book = new Book();
                book.ID = el.Attribute("id").Value;
                IEnumerable<XElement> props = el.Elements();
                foreach (XElement p in props)
                {


                    if (p.Name.ToString().ToLower() == "author")
                    {
                        book.Author = p.Value;
                    }
                    else if (p.Name.ToString().ToLower() == "title")
                    {
                        book.Title = p.Value;
                    }
                    else if (p.Name.ToString().ToLower() == "genre")
                    {
                        book.Genre = p.Value;
                    }
                    else if (p.Name.ToString().ToLower() == "price")
                    {
                        book.Price = Convert.ToDouble(p.Value);
                    }
                    else if (p.Name.ToString().ToLower() == "publish_date")
                    {
                        book.Publish_date = Convert.ToDateTime(p.Value);
                    }
                    else if (p.Name.ToString().ToLower() == "description")
                    {
                        book.Description = p.Value;
                    }
                }

                Books.Add(book);

            }

            DisplayResults(Books, "All books:");

        }

        // Explicit predicate delegate.
        private static bool FindComputer(Book bk)
        {

            if (bk.Genre == "Computer")
            {
                return true;
            }
        else
            {
                return false;
            }

        }

        private static void DisplayResult(Book result, string title)
        {
            Console.WriteLine();
            Console.WriteLine(title);
            Console.WriteLine("\n{0}\t{1}\t{2}\t{3}\t{4}\t{5}", result.ID,
                result.Author, result.Title, result.Genre, result.Price,
                result.Publish_date.ToShortDateString());
            Console.WriteLine();


        }

        private static void DisplayResults(List<Book> results, string title)
        {
            Console.WriteLine();
            Console.WriteLine(title);
            foreach (Book b in results)
            {

                Console.Write("\n{0}\t{1}\t{2}\t{3}\t{4}\t{5}", b.ID,
                    b.Author, b.Title, b.Genre, b.Price,
                    b.Publish_date.ToShortDateString());
            }
            Console.WriteLine();

        }

    }

    public class Book
    {
        public string ID { get; set; }
        public string Author { get; set; }
        public string Title { get; set; }
        public string Genre { get; set; }
        public double Price { get; set; }
        public DateTime Publish_date { get; set; }
        public string Description { get; set; }
    }
}
Imports System.Collections.Generic
Imports System.Linq
Imports System.Xml.Linq
Module Module1

    Private IDToFind As String = "bk109"

    Public Books As New List(Of Book)


    Sub Main()

        FillList()

        ' Find a book by its ID.
        Dim result As Book = Books.Find(AddressOf FindID)
        If result IsNot Nothing Then
            DisplayResult(result, "Find by ID: " & IDToFind)
        Else

            Console.WriteLine(vbCrLf & "Not found: " & IDToFind)
        End If
        Console.WriteLine()

        ' Find last book in collection that has a publish date before 2001.
        result = Books.FindLast(AddressOf PubBefore2001)
        If result IsNot Nothing Then
            DisplayResult(result, "Last book in collection published before 2001:")
        Else
            Console.WriteLine(vbCrLf & "Not found: " & IDToFind)
        End If
        Console.WriteLine()

        ' Find all computer books.
        Dim results As List(Of Book) = Books.FindAll(AddressOf FindComputer)
        If results.Count <> 0 Then
            DisplayResults(results, "All computer books:")
        Else
            Console.WriteLine(vbCrLf & "No books found.")
        End If
        Console.WriteLine()


        ' Find all books under $10.00.
        results = Books.FindAll(AddressOf FindUnderTen)
        If results.Count <> 0 Then
            DisplayResults(results, "Books under $10:")
        Else
            Console.WriteLine(vbCrLf & "No books found.")
        End If
        Console.WriteLine()

        ' Find index values.
        Console.WriteLine()
        Dim ndx As Integer = Books.FindIndex(AddressOf FindComputer)
        Console.WriteLine("Index of first computer book: " & ndx)
        ndx = Books.FindLastIndex(AddressOf FindComputer)
        Console.WriteLine("Index of last computer book: " & ndx)

        Dim mid As Integer = Books.Count / 2
        ndx = Books.FindIndex(mid, mid, AddressOf FindComputer)
        Console.WriteLine("Index of first computer book in the second half of the collection: " & ndx)



        ndx = Books.FindLastIndex(Books.Count - 1, mid, AddressOf FindComputer)
        Console.WriteLine("Index of last computer book in the second half of the collection: " & ndx)


    End Sub



    Private Sub FillList()

        ' Create XML elements from a source file.
        Dim xTree As XElement = XElement.Load("c:\temp\books.xml")

        ' Create an enumerable collection of the elements.
        Dim elements As IEnumerable(Of XElement) = xTree.Elements

        ' Evaluate each element and set values in the book object.
        For Each el As XElement In elements
            Dim Book As New Book()
            Book.ID = el.Attribute("id").Value
            Dim props As IEnumerable(Of XElement) = el.Elements
            For Each p As XElement In props
                If p.Name.ToString.ToLower = "author" Then
                    Book.Author = p.Value
                End If
                If p.Name.ToString.ToLower = "title" Then
                    Book.Title = p.Value
                End If
                If p.Name.ToString.ToLower = "genre" Then
                    Book.Genre = p.Value
                End If
                If p.Name.ToString.ToLower = "price" Then
                    Book.Price = Convert.ToDouble(p.Value)
                End If
                If p.Name.ToString.ToLower = "publish_date" Then
                    Book.Publish_date = Convert.ToDateTime(p.Value)
                End If
                If p.Name.ToString.ToLower = "description" Then
                    Book.Description = p.Value
                End If
            Next
            Books.Add(Book)
        Next

        DisplayResults(Books, "All books:")
        Console.WriteLine()

    End Sub

    ' Predicate delegates for
    ' Find and FindAll methods.
    Private Function FindID(ByVal bk As Book) As Boolean
        If bk.ID = IDToFind Then
            Return True
        Else
            Return False
        End If
    End Function
    Private Function FindComputer(ByVal bk As Book) As Boolean
        If bk.Genre = "Computer" Then
            Return True
        Else
            Return False
        End If
    End Function
    Private Function FindUnderTen(ByVal bk As Book) As Boolean
        Dim tendollars As Double = 10.0
        If bk.Price < tendollars Then
            Return True
        Else
            Return False
        End If
    End Function
    Private Function PubBefore2001(ByVal bk As Book) As Boolean
        Dim year2001 As DateTime = New DateTime(2001, 1, 1)
        Return bk.Publish_date < year2001
    End Function
    Private Sub DisplayResult(ByVal result As Book, ByVal title As String)
        Console.WriteLine()
        Console.WriteLine(title)
        Console.WriteLine(vbLf & result.ID & vbTab & result.Author & _
                          vbTab & result.Title & vbTab & result.Genre & _
                          vbTab & result.Publish_date & vbTab & result.Price)
        Console.WriteLine()
    End Sub
    Private Sub DisplayResults(ByVal results As List(Of Book), ByVal title As String)
        Console.WriteLine()
        Console.WriteLine(title)
        For Each b As Book In results
            Console.Write(vbLf & b.ID & vbTab & b.Author & _
                              vbTab & b.Title & vbTab & b.Genre & _
                              vbTab & b.Publish_date & vbTab & b.Price)
        Next
        Console.WriteLine()
    End Sub

    Public Class Book
        Public ID As String
        Public Author As String
        Public Title As String
        Public Genre As String
        Public Price As Double
        Public Publish_date As DateTime
        Public Description As String
    End Class


End Module

Hinweise

Der Predicate<T> ist ein Delegat für eine Methode, true die zurückgibt, wenn das an Sie übergebenen Objekt mit den im Delegaten definierten Bedingungen übereinstimmt.The Predicate<T> is a delegate to a method that returns true if the object passed to it matches the conditions defined in the delegate. Die Elemente der aktuellen List<T> werden einzeln an den Predicate<T> -Delegaten weitergegeben, wobei das List<T>-Element in den rückwärts verschoben wird, beginnend mit dem letzten-Element und endende mit dem ersten Element.The elements of the current List<T> are individually passed to the Predicate<T> delegate, moving backward in the List<T>, starting with the last element and ending with the first element. Die Verarbeitung wird beendet, wenn eine Entsprechung gefunden wird.Processing is stopped when a match is found.

Wichtig

Stellen Sie beim Durchsuchen einer Liste, die Werttypen enthält, sicher, dass der Standardwert für den Typ nicht das Such Prädikat erfüllt.When searching a list containing value types, make sure the default value for the type does not satisfy the search predicate. Andernfalls gibt es keine Möglichkeit, zwischen einem Standardwert zu unterscheiden, der angibt, dass keine Entsprechung gefunden wurde, und einem List-Element, das den Standardwert für den Typ hat.Otherwise, there is no way to distinguish between a default value indicating that no match was found and a list element that happens to have the default value for the type. Wenn der Standardwert das Such Prädikat erfüllt, verwenden Sie FindLastIndex stattdessen die-Methode.If the default value satisfies the search predicate, use the FindLastIndex method instead.

Diese Methode führt eine lineare Suche aus. Daher ist diese Methode ein O (n)-Vorgang, wobei n gleich Countist.This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

Gilt für:

Siehe auch