List<T>.FindAll(Predicate<T>) Method
Definition
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Retrieves all the elements that match the conditions defined by the specified predicate.
public:
System::Collections::Generic::List<T> ^ FindAll(Predicate<T> ^ match);public System.Collections.Generic.List<T> FindAll (Predicate<T> match);member this.FindAll : Predicate<'T> -> System.Collections.Generic.List<'T>Public Function FindAll (match As Predicate(Of T)) As List(Of T)Parameters
- match
- Predicate<T>
The Predicate<T> delegate that defines the conditions of the elements to search for.
Returns
A List<T> containing all the elements that match the conditions defined by the specified predicate, if found; otherwise, an empty List<T>.
Exceptions
match is null.
Examples
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.
| Method | Example |
|---|---|
| Find(Predicate<T>) | Finds a book by an ID using the IDToFind predicate delegate.C# example uses an anonymous delegate. |
| FindAll(Predicate<T>) | Find all books that whose Genre property is "Computer" using the FindComputer predicate delegate. |
| FindLast(Predicate<T>) | Finds the last book in the collection that has a publish date before 2001, using the PubBefore2001 predicate delegate.C# example uses an anonymous delegate. |
| FindIndex(Predicate<T>) | Finds the index of first computer book using the FindComputer predicate delegate. |
| FindLastIndex(Predicate<T>) | Finds the index of the last computer book using the FindComputer predicate delegate. |
| FindIndex(Int32, Int32, Predicate<T>) | Finds the index of first computer book in the second half of the collection, using the FindComputer predicate delegate. |
| FindLastIndex(Int32, Int32, Predicate<T>) | 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
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, and the elements that match the conditions are saved in the returned List<T>.
This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.
Applies to
See also
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