ICollection<T> Interface (System.Collections.Generic)

Overview

The ICollection<T> interface defines size, enumeration, and synchronization methods for all generic collections.

Syntax

public interface ICollection<T> : IEnumerable<T>, IEnumerable
{
    int Count { get; }
    bool IsReadOnly { get; }
    void Add(T item);
    void Clear();
    bool Contains(T item);
    void CopyTo(T[] array, int arrayIndex);
    bool Remove(T item);
}

Remarks

All generic collection classes in the System.Collections.Generic namespace implement ICollection<T>. It inherits from IEnumerable<T>, enabling foreach iteration.

The IsReadOnly property indicates whether the collection can be modified after creation.

Properties

Name	Type	Description
`Count`	int	The number of elements contained in the collection.
`IsReadOnly`	bool	True if the collection is read‑only; otherwise, false.

Methods

Signature	Description
`void Add(T item)`	Adds an item to the collection.
`void Clear()`	Removes all items from the collection.
`bool Contains(T item)`	Determines whether the collection contains a specific value.
`void CopyTo(T[] array, int arrayIndex)`	Copies the elements of the collection to an array, starting at a particular index.
`bool Remove(T item)`	Removes the first occurrence of a specific object from the collection.

Examples

The following example demonstrates common ICollection<T> operations using a List<int> which implements the interface.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        // Create a list that implements ICollection
        ICollection numbers = new List { 1, 2, 3, 4, 5 };

        Console.WriteLine("Count: " + numbers.Count);

        // Add a new element
        numbers.Add(6);
        Console.WriteLine("After Add(6): " + string.Join(", ", numbers));

        // Check if it contains a value
        Console.WriteLine("Contains 3? " + numbers.Contains(3));

        // Remove an element
        numbers.Remove(2);
        Console.WriteLine("After Remove(2): " + string.Join(", ", numbers));

        // Copy to array
        int[] array = new int[numbers.Count];
        numbers.CopyTo(array, 0);
        Console.WriteLine("Array copy: " + string.Join(", ", array));
    }
}