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

Definition

Determines whether the specified array contains elements that match the conditions defined by the specified predicate.

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

Type Parameters

T

The type of the elements of the array.

Parameters

array
T[]

The one-dimensional, zero-based Array to search.

match
Predicate<T>

The Predicate<T> that defines the conditions of the elements to search for.

Returns

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

Exceptions

array is null.

-or-

match is null.

Examples

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

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

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

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

Примечание

In C# and Visual Basic, it is not necessary to create the Predicate<T> delegate explicitly. These languages infer the correct delegate from context and create it automatically. In F#, functions and lambda expressions are implicitly converted.

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

Applies to

Продукт Версии
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1
UWP 10.0

See also