List(Of T).FindAll Method (Predicate(Of T))
Retrieves all the elements that match the conditions defined by the specified predicate.
Assembly: mscorlib (in mscorlib.dll)
Parameters
- match
-
Type:
System.Predicate(Of T)
The Predicate(Of T) delegate that defines the conditions of the elements to search for.
Return Value
Type: System.Collections.Generic.List(Of T)A List(Of T) containing all the elements that match the conditions defined by the specified predicate, if found; otherwise, an empty List(Of T).
| Exception | Condition |
|---|---|
| ArgumentNullException | match is null. |
The Predicate(Of 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(Of T) are individually passed to the Predicate(Of T) delegate, and the elements that match the conditions are saved in the returned List(Of T).
This method performs a linear search; therefore, this method is an O(n) operation, where n is Count.
The following example demonstrates the find methods for the List(Of T) class. The example for the List(Of T) class contains book objects, of class Book, using the data from the Sample XML File: Books (LINQ to XML)2. 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 |
|---|---|
Finds a book by an ID using the IDToFind predicate delegate. C# example uses an anonymous delegate. | |
FindAll(Predicate(Of T)) | Find all books that whose Genre property is "Computer" using the FindComputer predicate delegate. |
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. | |
Finds the index of first computer book using the FindComputer predicate delegate. | |
Finds the index of the last computer book using the FindComputer predicate delegate. | |
Finds the index of first computer book in the second half of the collection, using the FindComputer predicate delegate. | |
Finds the index of last computer book in the second half of the collection, using the FindComputer predicate delegate. |
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
Available since 8
.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1