List<T>.Exists(Predicate<T>) Method

Definition

Determines whether the List<T> contains elements that match the conditions defined by the specified predicate.

public bool Exists (Predicate<T> match);

Parameters

match
Predicate<T>

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

Returns

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

Exceptions

match is null.

Examples

The following example demonstrates the Contains and Exists methods on a List<T> that contains a simple business object that implements Equals.

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
         */
    }
}

The following example demonstrates the Exists method and several other methods that use the Predicate<T> generic delegate.

A List<T> of strings is created, containing 8 dinosaur names, two of which (at positions 1 and 5) end with "saurus". The example also defines a search predicate method named EndsWithSaurus, which accepts a string parameter and returns a Boolean value indicating whether the input string ends in "saurus".

The Find, FindLast, and FindAll methods are used to search the list with the search predicate method, and then the RemoveAll method is used to remove all entries ending with "saurus".

Finally, the Exists method is called. It traverses the list from the beginning, passing each element in turn to the EndsWithSaurus method. The search stops and the method returns true if the EndsWithSaurus method returns true for any element. The Exists method returns false because all such elements have been removed.

Note

In C# and Visual Basic, it is not necessary to create the Predicate<string> delegate (Predicate(Of String) in Visual Basic) explicitly. These languages infer the correct delegate from context and create it automatically.

using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        List<string> dinosaurs = new List<string>();

        dinosaurs.Add("Compsognathus");
        dinosaurs.Add("Amargasaurus");
        dinosaurs.Add("Oviraptor");
        dinosaurs.Add("Velociraptor");
        dinosaurs.Add("Deinonychus");
        dinosaurs.Add("Dilophosaurus");
        dinosaurs.Add("Gallimimus");
        dinosaurs.Add("Triceratops");

        Console.WriteLine();
        foreach(string dinosaur in dinosaurs)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\nTrueForAll(EndsWithSaurus): {0}",
            dinosaurs.TrueForAll(EndsWithSaurus));

        Console.WriteLine("\nFind(EndsWithSaurus): {0}",
            dinosaurs.Find(EndsWithSaurus));

        Console.WriteLine("\nFindLast(EndsWithSaurus): {0}",
            dinosaurs.FindLast(EndsWithSaurus));

        Console.WriteLine("\nFindAll(EndsWithSaurus):");
        List<string> sublist = dinosaurs.FindAll(EndsWithSaurus);

        foreach(string dinosaur in sublist)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine(
            "\n{0} elements removed by RemoveAll(EndsWithSaurus).",
            dinosaurs.RemoveAll(EndsWithSaurus));

        Console.WriteLine("\nList now contains:");
        foreach(string dinosaur in dinosaurs)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\nExists(EndsWithSaurus): {0}",
            dinosaurs.Exists(EndsWithSaurus));
    }

    // Search predicate returns true if a string ends in "saurus".
    private static bool EndsWithSaurus(String s)
    {
        return s.ToLower().EndsWith("saurus");
    }
}

/* This code example produces the following output:

Compsognathus
Amargasaurus
Oviraptor
Velociraptor
Deinonychus
Dilophosaurus
Gallimimus
Triceratops

TrueForAll(EndsWithSaurus): False

Find(EndsWithSaurus): Amargasaurus

FindLast(EndsWithSaurus): Dilophosaurus

FindAll(EndsWithSaurus):
Amargasaurus
Dilophosaurus

2 elements removed by RemoveAll(EndsWithSaurus).

List now contains:
Compsognathus
Oviraptor
Velociraptor
Deinonychus
Gallimimus
Triceratops

Exists(EndsWithSaurus): 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 the current List<T> are individually passed to the Predicate<T> delegate, and 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

Product Versions
.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, 1.6, 2.0, 2.1
UWP 10.0

See also