List<T> Constructors

Definition

Initializes a new instance of the List<T> class.

Overloads

List<T>()

Initializes a new instance of the List<T> class that is empty and has the default initial capacity.

List<T>(IEnumerable<T>)

Initializes a new instance of the List<T> class that contains elements copied from the specified collection and has sufficient capacity to accommodate the number of elements copied.

List<T>(Int32)

Initializes a new instance of the List<T> class that is empty and has the specified initial capacity.

List<T>()

Source:
List.cs
Source:
List.cs
Source:
List.cs

Initializes a new instance of the List<T> class that is empty and has the default initial capacity.

C#
public List();

Examples

The following example demonstrates the parameterless constructor of the List<T> generic class. The parameterless constructor creates a list with the default capacity, as demonstrated by displaying the Capacity property.

The example adds, inserts, and removes items, showing how the capacity changes as these methods are used.

C#
List<string> dinosaurs = new List<string>();

Console.WriteLine("\nCapacity: {0}", dinosaurs.Capacity);

dinosaurs.Add("Tyrannosaurus");
dinosaurs.Add("Amargasaurus");
dinosaurs.Add("Mamenchisaurus");
dinosaurs.Add("Deinonychus");
dinosaurs.Add("Compsognathus");
Console.WriteLine();
foreach(string dinosaur in dinosaurs)
{
    Console.WriteLine(dinosaur);
}

Console.WriteLine("\nCapacity: {0}", dinosaurs.Capacity);
Console.WriteLine("Count: {0}", dinosaurs.Count);

Console.WriteLine("\nContains(\"Deinonychus\"): {0}",
    dinosaurs.Contains("Deinonychus"));

Console.WriteLine("\nInsert(2, \"Compsognathus\")");
dinosaurs.Insert(2, "Compsognathus");

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

// Shows accessing the list using the Item property.
Console.WriteLine("\ndinosaurs[3]: {0}", dinosaurs[3]);

Console.WriteLine("\nRemove(\"Compsognathus\")");
dinosaurs.Remove("Compsognathus");

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

dinosaurs.TrimExcess();
Console.WriteLine("\nTrimExcess()");
Console.WriteLine("Capacity: {0}", dinosaurs.Capacity);
Console.WriteLine("Count: {0}", dinosaurs.Count);

dinosaurs.Clear();
Console.WriteLine("\nClear()");
Console.WriteLine("Capacity: {0}", dinosaurs.Capacity);
Console.WriteLine("Count: {0}", dinosaurs.Count);

/* This code example produces the following output:

Capacity: 0

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus

Capacity: 8
Count: 5

Contains("Deinonychus"): True

Insert(2, "Compsognathus")

Tyrannosaurus
Amargasaurus
Compsognathus
Mamenchisaurus
Deinonychus
Compsognathus

dinosaurs[3]: Mamenchisaurus

Remove("Compsognathus")

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus
Compsognathus

TrimExcess()
Capacity: 5
Count: 5

Clear()
Capacity: 5
Count: 0
 */

Remarks

The capacity of a List<T> is the number of elements that the List<T> can hold. As elements are added to a List<T>, the capacity is automatically increased as required by reallocating the internal array.

If the size of the collection can be estimated, using the List<T>(Int32) constructor and specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the List<T>.

The capacity can be decreased by calling the TrimExcess method or by setting the Capacity property explicitly. Decreasing the capacity reallocates memory and copies all the elements in the List<T>.

This constructor is an O(1) operation.

Applies to

.NET 9 and other versions
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

List<T>(IEnumerable<T>)

Source:
List.cs
Source:
List.cs
Source:
List.cs

Initializes a new instance of the List<T> class that contains elements copied from the specified collection and has sufficient capacity to accommodate the number of elements copied.

C#
public List(System.Collections.Generic.IEnumerable<T> collection);

Parameters

collection
IEnumerable<T>

The collection whose elements are copied to the new list.

Exceptions

collection is null.

Examples

The following example demonstrates the List<T> constructor and various methods of the List<T> class that act on ranges. An array of strings is created and passed to the constructor, populating the list with the elements of the array. The Capacity property is then displayed, to show that the initial capacity is exactly what is required to hold the input elements.

C#
using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        string[] input = { "Brachiosaurus",
                           "Amargasaurus",
                           "Mamenchisaurus" };

        List<string> dinosaurs = new List<string>(input);

        Console.WriteLine("\nCapacity: {0}", dinosaurs.Capacity);

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

        Console.WriteLine("\nAddRange(dinosaurs)");
        dinosaurs.AddRange(dinosaurs);

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

        Console.WriteLine("\nRemoveRange(2, 2)");
        dinosaurs.RemoveRange(2, 2);

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

        input = new string[] { "Tyrannosaurus",
                               "Deinonychus",
                               "Velociraptor"};

        Console.WriteLine("\nInsertRange(3, input)");
        dinosaurs.InsertRange(3, input);

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

        Console.WriteLine("\noutput = dinosaurs.GetRange(2, 3).ToArray()");
        string[] output = dinosaurs.GetRange(2, 3).ToArray();

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

/* This code example produces the following output:

Capacity: 3

Brachiosaurus
Amargasaurus
Mamenchisaurus

AddRange(dinosaurs)

Brachiosaurus
Amargasaurus
Mamenchisaurus
Brachiosaurus
Amargasaurus
Mamenchisaurus

RemoveRange(2, 2)

Brachiosaurus
Amargasaurus
Amargasaurus
Mamenchisaurus

InsertRange(3, input)

Brachiosaurus
Amargasaurus
Amargasaurus
Tyrannosaurus
Deinonychus
Velociraptor
Mamenchisaurus

output = dinosaurs.GetRange(2, 3).ToArray()

Amargasaurus
Tyrannosaurus
Deinonychus
 */

Remarks

The elements are copied onto the List<T> in the same order they are read by the enumerator of the collection.

This constructor is an O(n) operation, where n is the number of elements in collection.

See also

Applies to

.NET 9 and other versions
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

List<T>(Int32)

Source:
List.cs
Source:
List.cs
Source:
List.cs

Initializes a new instance of the List<T> class that is empty and has the specified initial capacity.

C#
public List(int capacity);

Parameters

capacity
Int32

The number of elements that the new list can initially store.

Exceptions

capacity is less than 0.

Examples

The following example demonstrates the List<T>(Int32) constructor. A List<T> of strings with a capacity of 4 is created, because the ultimate size of the list is known to be exactly 4. The list is populated with four strings, and a read-only copy is created by using the AsReadOnly method.

C#
using System;
using System.Collections.Generic;

public class Example
{
    public static void Main()
    {
        List<string> dinosaurs = new List<string>(4);

        Console.WriteLine("\nCapacity: {0}", dinosaurs.Capacity);

        dinosaurs.Add("Tyrannosaurus");
        dinosaurs.Add("Amargasaurus");
        dinosaurs.Add("Mamenchisaurus");
        dinosaurs.Add("Deinonychus");

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

        Console.WriteLine("\nIList<string> roDinosaurs = dinosaurs.AsReadOnly()");
        IList<string> roDinosaurs = dinosaurs.AsReadOnly();

        Console.WriteLine("\nElements in the read-only IList:");
        foreach(string dinosaur in roDinosaurs)
        {
            Console.WriteLine(dinosaur);
        }

        Console.WriteLine("\ndinosaurs[2] = \"Coelophysis\"");
        dinosaurs[2] = "Coelophysis";

        Console.WriteLine("\nElements in the read-only IList:");
        foreach(string dinosaur in roDinosaurs)
        {
            Console.WriteLine(dinosaur);
        }
    }
}

/* This code example produces the following output:

Capacity: 4

Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

IList<string> roDinosaurs = dinosaurs.AsReadOnly()

Elements in the read-only IList:
Tyrannosaurus
Amargasaurus
Mamenchisaurus
Deinonychus

dinosaurs[2] = "Coelophysis"

Elements in the read-only IList:
Tyrannosaurus
Amargasaurus
Coelophysis
Deinonychus
 */

Remarks

The capacity of a List<T> is the number of elements that the List<T> can hold. As elements are added to a List<T>, the capacity is automatically increased as required by reallocating the internal array.

If the size of the collection can be estimated, specifying the initial capacity eliminates the need to perform a number of resizing operations while adding elements to the List<T>.

The capacity can be decreased by calling the TrimExcess method or by setting the Capacity property explicitly. Decreasing the capacity reallocates memory and copies all the elements in the List<T>.

This constructor is an O(1) operation.

See also

Applies to

.NET 9 and other versions
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