C# Queue Tutorial

A Queue represents a first-in, first-out (FIFO) collection of objects. An example of a queue is a line of people waiting.

The Queue<T> class in the System.Collection.Generic namespace represents a queue in C#, where T specifies the type of elements in the queue. In this article and code examples, I explain the constructor, properties and methods of the Queue class and how to use its various methods using C#.

 

Queue Constructors 

 

The Queue<T> class has several overloaded constructors.

 

Queue<T> Constructor - Initializes a new instance of the Queue<T> class that is empty. The next lines shows how we create the empty queue.

Queue<string> queue = newQueue<string>();

Queue<T> Constructor (IEnumerable<T>) - Initializes a new instance of the Queue<T> class that contains elements copied from the specified collection.

string[] courses = { "MCA","MBA", "BCA","BBA", "BTech","MTech" };
Queue<string> queue = newQueue<string>(courses);

Queue<T> Constructor (Int32) - Initializes a new instance of the Queue<T> class that is empty and has the specified initial capacity.

Queue<string> queue = newQueue<string>(4);

Queue.Count Property

 

The Count property gets the number of elements of Queue<T>. The following code sample creates a queue of strings and gets the item count in the collection.

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            string[] courses = {
                "MCA",
                "MBA",
                "BCA",
                "BBA",
                "BTech",
                "MTech"
            };
            Queue < string > queue1 = newQueue < string > ();
            Queue < string > queue2 = newQueue < string > (courses);
            Queue < string > queue3 = newQueue < string > (4);
            Console.WriteLine("Number of elements in queue1:" + queue1.Count());
            Console.WriteLine("Number of elements in queue2:" + queue2.Count());
            Console.WriteLine("Number of elements in queue3:" + queue3.Count());
        }
    }
}

The output looks like this:

 

Queue.Enqueue Method

 

The Queue.Enqueue method adds an object to the end of the Queue<T>. For example, 

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            Queue < string > queue1 = newQueue < string > ();
            queue1.Enqueue("MCA");
            queue1.Enqueue("MBA");
            queue1.Enqueue("BCA");
            queue1.Enqueue("BBA");
            Console.WriteLine("The elements in the queue are:");
            foreach(string s in queue1) {
                Console.WriteLine(s);
            }
        }
    }
}

OUTPUT 

 

Queue.Dequeue() method

 

Removes and returns the object at the beginning of the Queue<T>. For example, 

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            Queue < string > queue1 = newQueue < string > ();
            queue1.Enqueue("MCA");
            queue1.Enqueue("MBA");
            queue1.Enqueue("BCA");
            queue1.Enqueue("BBA");
            Console.WriteLine("The elements in the queue are:");
            foreach(string s in queue1) {
                Console.WriteLine(s);
            }
            queue1.Dequeue(); //Removes the first element that enter in the queue here the first element is MCA
            queue1.Dequeue(); //Removes MBA
            Console.WriteLine("After removal the elements in the queue are:");
            foreach(string s in queue1) {
                Console.WriteLine(s);
            }
        }
    }
}

OUTPUT

 

 

Queue.Contain() method

 

Determines whether an element is in the Queue<T>. For example,

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            Queue < string > queue1 = newQueue < string > ();
            queue1.Enqueue("MCA");
            queue1.Enqueue("MBA");
            queue1.Enqueue("BCA");
            queue1.Enqueue("BBA");
            Console.WriteLine("The elements in the queue are:");
            foreach(string s in queue1) {
                Console.WriteLine(s);
            }
            Console.WriteLine("The element MCA is contain in the queue:" + queue1.Contains("MCA"));
            Console.WriteLine("The element BCA is contain in the queue:" + queue1.Contains("BCA"));
            Console.WriteLine("The element MTech is contain in the queue:" + queue1.Contains("MTech"));
        }
    }
}

OUTPUT

 

Queue.Clear() method

 

Removes all objects from theQueue<T>. For example,

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            Queue < string > queue1 = newQueue < string > ();
            queue1.Enqueue("MCA");
            queue1.Enqueue("MBA");
            queue1.Enqueue("BCA");
            queue1.Enqueue("BBA");
            Console.WriteLine("The elements in the queue are:" + queue1.Count());
            queue1.Clear();
            Console.WriteLine("The elements in the queue are after the clear method:" + queue1.Count());
        }
    }
}

OUTPUT

 

Queue.Peek method

 

Returns the object at the beginning of the Queue<T> without removing it. For example,

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            Queue < string > queue1 = newQueue < string > ();
            queue1.Enqueue("MCA");
            queue1.Enqueue("MBA");
            queue1.Enqueue("BCA");
            queue1.Enqueue("BBA");
            Console.WriteLine("Peek the first item from the queue is:" + queue1.Peek());
            queue1.Dequeue();
            Console.WriteLine("Peek the next item from the queue is:" + queue1.Peek());
        }
    }
}

OUTPUT

 

Queue.ToArray() method

 

Copies the Queue<T> elements to a new array. For example,

namespace Queue {
    classProgram {
        staticvoid Main(string[] args) {
            Queue < string > queue1 = newQueue < string > ();
            queue1.Enqueue("MCA");
            queue1.Enqueue("MBA");
            queue1.Enqueue("BCA");
            queue1.Enqueue("BBA");
            Console.WriteLine("The queue elements are:");
            foreach(string s in queue1) {
                Console.WriteLine(s);
            }
            Queue < string > queue2 = newQueue < string > (queue1.ToArray());
            Console.WriteLine("\nContents of the copy");
            foreach(string n in queue2) {
                Console.WriteLine(n);
            }
        }
    }
}

OUTPUT

 

 

Summary

 

In this article I explained the Queue<> class and the various methods and properties of it.

 

Here are recommended articles on collections:

1. C# Dictionary
2. C# Queue
3. C# Stack
4. C# List
5. C# Arrays
6. C# HashTable
7. C# StringCollection
8. C# ArrayList
9. C# HashSet
10. C# LinkedList