List<T>.Find(Predicate<T>) Method (System.Collections.Generic) (2023)

public: T Find(Predicate<T> ^ match);

public T Find (Predicate<T> match);

public T? Find (Predicate<T> match);

member this.Find : Predicate<'T> -> 'T

Public Function Find (match As Predicate(Of T)) As T

Parameters

Returns

T

The first element that matches the conditions defined by the specified predicate, if found; otherwise, the default value for type T.

Exceptions

Examples

The following example demonstrates the Find method on a List<T> that contains a simple complex object.

using System;using System.Collections.Generic;// Simple business object. A PartId is used to identify a part// but the part name can change.public class Part : IEquatable<Part>{ public string PartName { get; set; } public int PartId { get; set; } public override string ToString() { return "ID: " + PartId + " Name: " + PartName; } public override bool Equals(object obj) { if (obj == null) return false; Part objAsPart = obj as Part; if (objAsPart == null) return false; else return Equals(objAsPart); } public override int GetHashCode() { return PartId; } public bool Equals(Part other) { if (other == null) return false; return (this.PartId.Equals(other.PartId)); } // Should also override == and != operators.}public class Example{ public static void Main() { // Create a list of parts. List<Part> parts = new List<Part>(); // Add parts to the list. parts.Add(new Part() { PartName = "crank arm", PartId = 1234 }); parts.Add(new Part() { PartName = "chain ring", PartId = 1334 }); parts.Add(new Part() { PartName = "regular seat", PartId = 1434 }); parts.Add(new Part() { PartName = "banana seat", PartId = 1444 }); parts.Add(new Part() { PartName = "cassette", PartId = 1534 }); parts.Add(new Part() { PartName = "shift lever", PartId = 1634 }); ; // Write out the parts in the list. This will call the overridden ToString method // in the Part class. Console.WriteLine(); foreach (Part aPart in parts) { Console.WriteLine(aPart); } // Check the list for part #1734. This calls the IEquatable.Equals method // of the Part class, which checks the PartId for equality. Console.WriteLine("\nContains: Part with Id=1734: {0}", parts.Contains(new Part { PartId = 1734, PartName = "" })); // Find items where name contains "seat". Console.WriteLine("\nFind: Part where name contains \"seat\": {0}", parts.Find(x => x.PartName.Contains("seat"))); // Check if an item with Id 1444 exists. Console.WriteLine("\nExists: Part with Id=1444: {0}", parts.Exists(x => x.PartId == 1444)); /*This code example produces the following output: ID: 1234 Name: crank arm ID: 1334 Name: chain ring ID: 1434 Name: regular seat ID: 1444 Name: banana seat ID: 1534 Name: cassette ID: 1634 Name: shift lever Contains: Part with Id=1734: False Find: Part where name contains "seat": ID: 1434 Name: regular seat Exists: Part with Id=1444: True */ }}

Imports System.Collections.Generic' Simple business object. A PartId is used to identify a part ' but the part name can change. Public Class Part Implements IEquatable(Of Part) Public Property PartName() As String Get Return m_PartName End Get Set(value As String) m_PartName = Value End Set End Property Private m_PartName As String Public Property PartId() As Integer Get Return m_PartId End Get Set(value As Integer) m_PartId = Value End Set End Property Private m_PartId As Integer Public Overrides Function ToString() As String Return Convert.ToString("ID: " & PartId & " Name: ") & PartName End Function Public Overrides Function Equals(obj As Object) As Boolean If obj Is Nothing Then Return False End If Dim objAsPart As Part = TryCast(obj, Part) If objAsPart Is Nothing Then Return False Else Return Equals(objAsPart) End If End Function Public Overrides Function GetHashCode() As Integer Return PartId End Function Public Overloads Function Equals(other As Part) As Boolean _ Implements IEquatable(Of Part).Equals If other Is Nothing Then Return False End If Return (Me.PartId.Equals(other.PartId)) End Function ' Should also override == and != operators.End ClassPublic Class Example Public Shared Sub Main() ' Create a list of parts. Dim parts As New List(Of Part)() ' Add parts to the list. parts.Add(New Part() With { _ .PartName = "crank arm", _ .PartId = 1234 _ }) parts.Add(New Part() With { _ .PartName = "chain ring", _ .PartId = 1334 _ }) parts.Add(New Part() With { _ .PartName = "regular seat", _ .PartId = 1434 _ }) parts.Add(New Part() With { _ .PartName = "banana seat", _ .PartId = 1444 _ }) parts.Add(New Part() With { _ .PartName = "cassette", _ .PartId = 1534 _ }) parts.Add(New Part() With { _ .PartName = "shift lever", _ .PartId = 1634 _ }) ' Write out the parts in the list. This will call the overridden ToString method ' in the Part class. Console.WriteLine() For Each aPart As Part In parts Console.WriteLine(aPart) Next ' Check the list for part #1734. This calls the IEquatable.Equals method ' of the Part class, which checks the PartId for equality. Console.WriteLine(vbLf & "Contains: Part with Id=1734: {0}", parts.Contains(New Part() With { _ .PartId = 1734, _ .PartName = "" _ })) ' Find items where name contains "seat". Console.WriteLine(vbLf & "Find: Part where name contains ""seat"": {0}", parts.Find(Function(x) x.PartName.Contains("seat"))) ' Check if an item with Id 1444 exists. Console.WriteLine(vbLf & "Exists: Part with Id=1444: {0}", parts.Exists(Function(x) x.PartId = 1444)) 'This code example produces the following output: ' ' ID: 1234 Name: crank arm ' ID: 1334 Name: chain ring ' ID: 1434 Name: regular seat ' ID: 1444 Name: banana seat ' ID: 1534 Name: cassette ' ID: 1634 Name: shift lever ' ' Contains: Part with Id=1734: False ' ' Find: Part where name contains "seat": ID: 1434 Name: regular seat ' ' Exists: Part with Id=1444: True ' End SubEnd Class

The following example demonstrates the find methods for the List<T> class. 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). The FillList method in the example uses LINQ to XML to parse the values from the XML to property values of the book objects.

The following table describes the examples provided for the find methods.

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.GenericImports System.LinqImports System.Xml.LinqModule 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 ClassEnd Module

Remarks

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. The elements of the current List<T> are individually passed to the Predicate<T> delegate, moving forward in the List<T>, starting with the first element and ending with the last element. Processing is stopped when a match is found.

This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.

Applies to

See also

• Exists(Predicate<T>)
• FindLast(Predicate<T>)
• FindAll(Predicate<T>)
• FindIndex
• FindLastIndex
• BinarySearch
• IndexOf
• LastIndexOf
• Predicate<T>