Array.Exists<T>(T[], Predicate<T>) Method

public:
generic <typename T>
 static bool Exists(cli::array <T> ^ array, Predicate<T> ^ match);

public static bool Exists<T>(T[] array, Predicate<T> match);

static member Exists : 'T[] * Predicate<'T> -> bool

Public Shared Function Exists(Of T) (array As T(), match As Predicate(Of T)) As Boolean

true if array contains one or more elements that match the conditions defined by the specified predicate; otherwise, false.

The following example specifies the match conditions for the Exists method using lambda expressions to check whether a planet starts with a given letter or whether the planet is found on the given array.

using System;

namespace Example
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] planets = { "Mercury", "Venus",
                "Earth", "Mars", "Jupiter",
                "Saturn", "Uranus", "Neptune" };

            Console.WriteLine("One or more planets begin with 'M': {0}",
                Array.Exists(planets, element => element.StartsWith("M")));

            Console.WriteLine("One or more planets begin with 'T': {0}",
                Array.Exists(planets, element => element.StartsWith("T")));

            Console.WriteLine("Is Pluto one of the planets? {0}",
                Array.Exists(planets, element => element == "Pluto"));
        }
    }
}
// The example displays the following output:
//       One or more planets begin with 'M': True
//       One or more planets begin with 'T': False
//       Is Pluto one of the planets? False

open System

let  planets = 
    [| "Mercury"; "Venus"
       "Earth"; "Mars"; "Jupiter"
       "Saturn"; "Uranus"; "Neptune" |]

Array.Exists(planets, fun element -> element.StartsWith "M")
|> printfn "One or more planets begin with 'M': %O"

Array.Exists(planets, fun element -> element.StartsWith "T")
|> printfn "One or more planets begin with 'T': %O"

Array.Exists(planets, fun element -> element = "Pluto")
|> printfn "Is Pluto one of the planets? %O"

// The example displays the following output:
//       One or more planets begin with 'M': True
//       One or more planets begin with 'T': False
//       Is Pluto one of the planets? False

Module Example
    Public Sub Main()
        Dim planets() As String = {"Mercury", "Venus",
                                    "Earth", "Mars", "Jupiter",
                                    "Saturn", "Uranus", "Neptune"}

        Console.WriteLine("One or more planets begin with 'M': {0}",
            Array.Exists(planets, Function(element)
                                      Return element.StartsWith("M")
                                  End Function))

        Console.WriteLine("One or more planets begin with 'T': {0}",
            Array.Exists(planets, Function(element)
                                      Return element.StartsWith("T")
                                  End Function))

        Console.WriteLine("Is Pluto one of the planets? {0}",
            Array.Exists(planets, Function(element)
                                      Return element.Equals("Pluto")
                                  End Function))

    End Sub
End Module
' The example displays the following output:
'       One or more planets begin with 'M': True
'       One or more planets begin with 'T': False
'       Is Pluto one of the planets? False

The following example uses the Exists method to indicate whether any names in a string array begin with a specified character. The example instantiates a StringSearcher object by passing the string to search for to its class constructor. The StringSearcher.StartsWith method has same signature as the Predicate<T> delegate. When the Exists method is called, each member of the array is passed to the delegate until it returns true or iterates all the elements in the array.

using System;

public class Example
{
   public static void Main()
   {
      String[] names = { "Adam", "Adel", "Bridgette", "Carla",
                         "Charles", "Daniel", "Elaine", "Frances",
                         "George", "Gillian", "Henry", "Irving",
                         "James", "Janae", "Lawrence", "Miguel",
                         "Nicole", "Oliver", "Paula", "Robert",
                         "Stephen", "Thomas", "Vanessa",
                         "Veronica", "Wilberforce" };
      Char[] charsToFind = { 'A', 'K', 'W', 'Z' };

      foreach (var charToFind in charsToFind)
         Console.WriteLine("One or more names begin with '{0}': {1}",
                           charToFind,
                           Array.Exists(names, (new StringSearcher(charToFind)).StartsWith));
   }
}

public class StringSearcher
{
   char firstChar;

   public StringSearcher(char firstChar)
   {
      this.firstChar = char.ToUpper(firstChar);
   }

   public bool StartsWith(string s)
   {
      if (string.IsNullOrEmpty(s)) return false;

      if(s.Substring(0, 1).ToUpper() == firstChar.ToString())
         return true;
      else
         return false;
   }
}
// The example displays the following output:
//       One or more names begin with 'A': True
//       One or more names begin with 'K': False
//       One or more names begin with 'W': True
//       One or more names begin with 'Z': False

open System

type StringSearcher(firstChar) =
    member _.StartsWith(s) =
        if String.IsNullOrEmpty s then 
            false 
        else
            s.Substring(0, 1).ToUpper() = string firstChar

let names = 
    [| "Adam"; "Adel"; "Bridgette"; "Carla";
       "Charles"; "Daniel"; "Elaine"; "Frances"
       "George"; "Gillian"; "Henry"; "Irving"
       "James"; "Janae"; "Lawrence"; "Miguel"
       "Nicole"; "Oliver"; "Paula"; "Robert"
       "Stephen"; "Thomas"; "Vanessa"
       "Veronica"; "Wilberforce" |]

let charsToFind = [ 'A'; 'K'; 'W'; 'Z' ]

for char in charsToFind do 
    let exists = Array.Exists(names, fun x -> StringSearcher(char).StartsWith x)
    // let exists = Array.exists (StringSearcher(char).StartsWith) names
    printfn $"One or more names begin with '{char}': {exists}"

// The example displays the following output:
//       One or more names begin with 'A': True
//       One or more names begin with 'K': False
//       One or more names begin with 'W': True
//       One or more names begin with 'Z': False

Module Example
   Public Sub Main()
      Dim names() As String = { "Adam", "Adel", "Bridgette", "Carla",
                                "Charles", "Daniel", "Elaine", "Frances",
                                "George", "Gillian", "Henry", "Irving",
                                "James", "Janae", "Lawrence", "Miguel",
                                "Nicole", "Oliver", "Paula", "Robert",
                                "Stephen", "Thomas", "Vanessa",
                                "Veronica", "Wilberforce" }
      Dim charsToFind() As Char = { "A"c, "K"c, "W"c, "Z"c }
      
      For Each charToFind In charsToFind
         Console.WriteLine("One or more names begin with '{0}': {1}",
                           charToFind,
                           Array.Exists(names, AddressOf (New StringSearcher(charToFind)).StartsWith))
      Next
   End Sub
   
End Module

Public Class StringSearcher
   Dim firstChar As Char
   
   Public Sub New(firstChar As Char)
      Me.firstChar = Char.ToUpper(firstChar)
   End Sub
   
   Public Function StartsWith(s As String) As Boolean
      If String.IsNullOrEmpty(s) Then Return False
      
      If s.Substring(0, 1).ToUpper = firstChar Then
         Return True
      Else
         Return False
      End If
   End Function
End Class
' The example displays the following output:
'       One or more names begin with 'A': True
'       One or more names begin with 'K': False
'       One or more names begin with 'W': True
'       One or more names begin with 'Z': False

You can also use a lambda expression rather than explicitly define a method whose signature corresponds to that of the delegate. The following example replaces the StringSearcher class and its StartsWith method with a lambda expression.

using System;

public class Example
{
   public static void Main()
   {
      String[] names = { "Adam", "Adel", "Bridgette", "Carla",
                         "Charles", "Daniel", "Elaine", "Frances",
                         "George", "Gillian", "Henry", "Irving",
                         "James", "Janae", "Lawrence", "Miguel",
                         "Nicole", "Oliver", "Paula", "Robert",
                         "Stephen", "Thomas", "Vanessa",
                         "Veronica", "Wilberforce" };
      Char[] charsToFind = { 'A', 'K', 'W', 'Z' };

      foreach (var charToFind in charsToFind)
         Console.WriteLine("One or more names begin with '{0}': {1}",
                           charToFind,
                           Array.Exists(names,
                                        s => { if (string.IsNullOrEmpty(s))
                                                  return false;

                                               if (s.Substring(0, 1).ToUpper() == charToFind.ToString())
                                                  return true;
                                               else
                                                  return false;
                                             } ));
   }
}
// The example displays the following output:
//       One or more names begin with 'A': True
//       One or more names begin with 'K': False
//       One or more names begin with 'W': True
//       One or more names begin with 'Z': False

open System

let names = 
    [| "Adam"; "Adel"; "Bridgette"; "Carla";
       "Charles"; "Daniel"; "Elaine"; "Frances"
       "George"; "Gillian"; "Henry"; "Irving"
       "James"; "Janae"; "Lawrence"; "Miguel"
       "Nicole"; "Oliver"; "Paula"; "Robert"
       "Stephen"; "Thomas"; "Vanessa"
       "Veronica"; "Wilberforce" |]

let charsToFind = [ 'A'; 'K'; 'W'; 'Z' ]

for char in charsToFind do
    let exists = 
        Array.Exists(names, fun s -> 
            if String.IsNullOrEmpty s then false 
            else s.Substring(0, 1).ToUpper() = string char)

    printfn $"One or more names begin with '{char}': {exists}"
                    
// The example displays the following output:
//       One or more names begin with 'A': True
//       One or more names begin with 'K': False
//       One or more names begin with 'W': True
//       One or more names begin with 'Z': False

Module Example
   Public Sub Main()
      Dim names() As String = { "Adam", "Adel", "Bridgette", "Carla",
                                "Charles", "Daniel", "Elaine", "Frances",
                                "George", "Gillian", "Henry", "Irving",
                                "James", "Janae", "Lawrence", "Miguel",
                                "Nicole", "Oliver", "Paula", "Robert",
                                "Stephen", "Thomas", "Vanessa",
                                "Veronica", "Wilberforce" }
      Dim charsToFind() As Char = { "A"c, "K"c, "W"c, "Z"c }

      For Each charToFind In charsToFind
         Console.WriteLine("One or more names begin with '{0}': {1}",
                           charToFind,
                           Array.Exists(names, Function(s)
                                                  If String.IsNullOrEmpty(s) Then Return False

                                                  If s.Substring(0, 1).ToUpper = charToFind Then
                                                     Return True
                                                  Else
                                                     Return False
                                                  End If
                                               End Function ))
      Next
   End Sub
End Module
' The example displays the following output:
'       One or more names begin with 'A': True
'       One or more names begin with 'K': False
'       One or more names begin with 'W': True
'       One or more names begin with 'Z': False

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 array are individually passed to the Predicate<T>, and processing is stopped when a match is found.

This method is an O(n) operation, where n is the Length of array.

• Find<T>(T[], Predicate<T>)
• FindLast<T>(T[], Predicate<T>)
• FindAll<T>(T[], Predicate<T>)
• BinarySearch
• IndexOf
• LastIndexOf
• TrueForAll<T>(T[], Predicate<T>)
• Predicate<T>
• Lambda Expressions (C# Programming Guide)
• Lambda Expressions (Visual Basic)