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Smaller performance tasks following each reading within collections also provide opportunities to integrate knowledge and ideas from the texts they are reading. Below is an example of tasks from Grade 6, Collection 2: Informative essay: a one-page essay about a character from the story, “The Mixer”. Find Home Decor, Gifts & Apparel, and Seasonal decorations at Collections Etc. We offer over 3,000 items $14.99 or less.
For many applications, you want to create and manage groups of related objects. There are two ways to group objects: by creating arrays of objects, and by creating collections of objects.
Arrays are most useful for creating and working with a fixed number of strongly typed objects. For information about arrays, see Arrays.
Collections provide a more flexible way to work with groups of objects. Unlike arrays, the group of objects you work with can grow and shrink dynamically as the needs of the application change. For some collections, you can assign a key to any object that you put into the collection so that you can quickly retrieve the object by using the key.
A collection is a class, so you must declare an instance of the class before you can add elements to that collection.
If your collection contains elements of only one data type, you can use one of the classes in the System.Collections.Generic namespace. A generic collection enforces type safety so that no other data type can be added to it. When you retrieve an element from a generic collection, you do not have to determine its data type or convert it.
Note
For the examples in this topic, include Imports statements for the
System.Collections.Generic
and System.Linq
namespaces.Using a Simple Collection
The examples in this section use the generic List<T> class, which enables you to work with a strongly typed list of objects.
The following example creates a list of strings and then iterates through the strings by using a For Each…Next statement.
If the contents of a collection are known in advance, you can use a collection initializer to initialize the collection. For more information, see Collection Initializers.
The following example is the same as the previous example, except a collection initializer is used to add elements to the collection.
You can use a For…Next statement instead of a
For Each
statement to iterate through a collection. You accomplish this by accessing the collection elements by the index position. The index of the elements starts at 0 and ends at the element count minus 1.The following example iterates through the elements of a collection by using
For…Next
instead of For Each
.The following example removes an element from the collection by specifying the object to remove.
The following example removes elements from a generic list. Instead of a
For Each
statement, a For…Next statement that iterates in descending order is used. This is because the RemoveAt method causes elements after a removed element to have a lower index value.For the type of elements in the List<T>, you can also define your own class. In the following example, the
Galaxy
class that is used by the List<T> is defined in the code.Kinds of Collections
Many common collections are provided by the .NET Framework. Each type of collection is designed for a specific purpose.
Some of the common collection classes are described in this section:
- System.Collections.Generic classes
- System.Collections.Concurrent classes
- System.Collections classes
- Visual Basic
Collection
class
System.Collections.Generic Classes
You can create a generic collection by using one of the classes in the System.Collections.Generic namespace. A generic collection is useful when every item in the collection has the same data type. A generic collection enforces strong typing by allowing only the desired data type to be added.
The following table lists some of the frequently used classes of the System.Collections.Generic namespace:
Class | Description |
---|---|
Dictionary<TKey,TValue> | Represents a collection of key/value pairs that are organized based on the key. |
List<T> | Represents a list of objects that can be accessed by index. Provides methods to search, sort, and modify lists. |
Queue<T> | Represents a first in, first out (FIFO) collection of objects. |
SortedList<TKey,TValue> | Represents a collection of key/value pairs that are sorted by key based on the associated IComparer<T> implementation. |
Stack<T> | Represents a last in, first out (LIFO) collection of objects. |
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For additional information, see Commonly Used Collection Types, Selecting a Collection Class, and System.Collections.Generic.
System.Collections.Concurrent Classes
In the .NET Framework 4 or newer, the collections in the System.Collections.Concurrent namespace provide efficient thread-safe operations for accessing collection items from multiple threads.
The classes in the System.Collections.Concurrent namespace should be used instead of the corresponding types in the System.Collections.Generic and System.Collections namespaces whenever multiple threads are accessing the collection concurrently. For more information, see Thread-Safe Collections and System.Collections.Concurrent.
Some classes included in the System.Collections.Concurrent namespace are BlockingCollection<T>, ConcurrentDictionary<TKey,TValue>, ConcurrentQueue<T>, and ConcurrentStack<T>.
System.Collections Classes
The classes in the System.Collections namespace do not store elements as specifically typed objects, but as objects of type
Object
.Whenever possible, you should use the generic collections in the System.Collections.Generic namespace or the System.Collections.Concurrent namespace instead of the legacy types in the
System.Collections
namespace.The following table lists some of the frequently used classes in the
System.Collections
namespace:Class | Description |
---|---|
ArrayList | Represents an array of objects whose size is dynamically increased as required. |
Hashtable | Represents a collection of key/value pairs that are organized based on the hash code of the key. |
Queue | Represents a first in, first out (FIFO) collection of objects. |
Stack | Represents a last in, first out (LIFO) collection of objects. |
The System.Collections.Specialized namespace provides specialized and strongly typed collection classes, such as string-only collections and linked-list and hybrid dictionaries.
Visual Basic Collection Class
You can use the Visual Basic Collection class to access a collection item by using either a numeric index or a
String
key. You can add items to a collection object either with or without specifying a key. If you add an item without a key, you must use its numeric index to access it.The Visual Basic
Collection
class stores all its elements as type Object
, so you can add an item of any data type. There is no safeguard against inappropriate data types being added.When you use the Visual Basic
Collection
class, the first item in a collection has an index of 1. This differs from the .NET Framework collection classes, for which the starting index is 0.Whenever possible, you should use the generic collections in the System.Collections.Generic namespace or the System.Collections.Concurrent namespace instead of the Visual Basic
Collection
class.For more information, see Collection.
Implementing a Collection of Key/Value Pairs
The Dictionary<TKey,TValue> generic collection enables you to access to elements in a collection by using the key of each element. Each addition to the dictionary consists of a value and its associated key. Retrieving a value by using its key is fast because the
Dictionary
class is implemented as a hash table.The following example creates a
Dictionary
collection and iterates through the dictionary by using a For Each
statement.To instead use a collection initializer to build the
Dictionary
collection, you can replace the BuildDictionary
and AddToDictionary
Omnigraffle pro 7 13 laptop. methods with the following method.The following example uses the ContainsKey method and the Item[] property of
Dictionary
to quickly find an item by key. The Item
property enables you to access an item in the elements
collection by using the elements(symbol)
code in Visual Basic.The following example instead uses the TryGetValue method quickly find an item by key.
Using LINQ to Access a Collection
LINQ (Language-Integrated Query) can be used to access collections. LINQ queries provide filtering, ordering, and grouping capabilities. For more information, see Getting Started with LINQ in Visual Basic.
The following example runs a LINQ query against a generic
List
. The LINQ query returns a different collection that contains the results.Sorting a Collection
The following example illustrates a procedure for sorting a collection. The example sorts instances of the
Car
class that are stored in a List<T>. The Car
class implements the IComparable<T> interface, which requires that the CompareTo method be implemented.Each call to the CompareTo method makes a single comparison that is used for sorting. User-written code in the
CompareTo
method returns a value for each comparison of the current object with another object. The value returned is less than zero if the current object is less than the other object, greater than zero if the current object is greater than the other object, and zero if they are equal. This enables you to define in code the criteria for greater than, less than, and equal.Collections 6 2 2018
In the
ListCars
method, the cars.Sort()
statement sorts the list. This call to the Sort method of the List<T> causes the CompareTo
method to be called automatically for the Car
objects in the List
.Defining a Custom Collection
You can define a collection by implementing the IEnumerable<T> or IEnumerable interface. For additional information, see Enumerating a Collection.
Although you can define a custom collection, it is usually better to instead use the collections that are included in the .NET Framework, which are described in Kinds of Collections earlier in this topic.
The following example defines a custom collection class named
AllColors
. This class implements the IEnumerable interface, which requires that the GetEnumerator method be implemented.The
GetEnumerator
method returns an instance of the ColorEnumerator
class. ColorEnumerator
implements the IEnumerator interface, which requires that the Current property, MoveNext method, and Reset method be implemented.Iterators
An iterator is used to perform a custom iteration over a collection. An iterator can be a method or a
get
accessor. An iterator uses a Yield statement to return each element of the collection one at a time.You call an iterator by using a For Each…Next statement. Each iteration of the
For Each
loop calls the iterator. When a Yield
statement is reached in the iterator, an expression is returned, and the current location in code is retained. Execution is restarted from that location the next time that the iterator is called.For more information, see Iterators (Visual Basic).
The following example uses an iterator method. The iterator method has a
Yield
statement that is inside a For…Next loop. In the ListEvenNumbers
method, each iteration of the For Each
statement body creates a call to the iterator method, which proceeds to the next Yield
statement.