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Collections (C# and Visual Basic)

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 in Visual Basic or Arrays (C# Programming Guide).

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 a new collection 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 Note

For the examples in this topic, include Imports statements (Visual Basic) or using directives (C#) for the System.Collections.Generic and System.Linq namespaces.

In this topic

The examples in this section use the generic List(Of 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 (Visual Basic) or foreach (C#) statement.

' Create a list of strings. 
Dim salmons As New List(Of String)
salmons.Add("chinook")
salmons.Add("coho")
salmons.Add("pink")
salmons.Add("sockeye")

' Iterate through the list. 
For Each salmon As String In salmons
    Console.Write(salmon & " ")
Next 
'Output: chinook coho pink sockeye

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 (Visual Basic) or Object and Collection Initializers (C# Programming Guide).

The following example is the same as the previous example, except a collection initializer is used to add elements to the collection.

' Create a list of strings by using a 
' collection initializer. 
Dim salmons As New List(Of String) From
    {"chinook", "coho", "pink", "sockeye"}

For Each salmon As String In salmons
    Console.Write(salmon & " ")
Next 
'Output: chinook coho pink sockeye

You can use a For…Next (Visual Basic) or for (C#) 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.

Dim salmons As New List(Of String) From
    {"chinook", "coho", "pink", "sockeye"}

For index = 0 To salmons.Count - 1
    Console.Write(salmons(index) & " ")
Next 
'Output: chinook coho pink sockeye

The following example removes an element from the collection by specifying the object to remove.

' Create a list of strings by using a 
' collection initializer. 
Dim salmons As New List(Of String) From
    {"chinook", "coho", "pink", "sockeye"}

' Remove an element in the list by specifying 
' the object.
salmons.Remove("coho")

For Each salmon As String In salmons
    Console.Write(salmon & " ")
Next 
'Output: chinook pink sockeye

The following example removes elements from a generic list. Instead of a For Each statement, a For…Next (Visual Basic) or for (C#) 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.

Dim numbers As New List(Of Integer) From
    {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}

' Remove odd numbers. 
For index As Integer = numbers.Count - 1 To 0 Step -1
    If numbers(index) Mod 2 = 1 Then 
        ' Remove the element by specifying 
        ' the zero-based index in the list.
        numbers.RemoveAt(index)
    End If 
Next 

' Iterate through the list. 
' A lambda expression is placed in the ForEach method 
' of the List(T) object.
numbers.ForEach(
    Sub(number) Console.Write(number & " "))
' Output: 0 2 4 6 8

For the type of elements in the List(Of T), you can also define your own class. In the following example, the Galaxy class that is used by the List(Of T) is defined in the code.

Private Sub IterateThroughList()
    Dim theGalaxies As New List(Of Galaxy) From
        {
            New Galaxy With {.Name = "Tadpole", .MegaLightYears = 400},
            New Galaxy With {.Name = "Pinwheel", .MegaLightYears = 25},
            New Galaxy With {.Name = "Milky Way", .MegaLightYears = 0},
            New Galaxy With {.Name = "Andromeda", .MegaLightYears = 3}
        }

    For Each theGalaxy In theGalaxies
        With theGalaxy
            Console.WriteLine(.Name & "  " & .MegaLightYears)
        End With 
    Next 

    ' Output: 
    '  Tadpole  400 
    '  Pinwheel  25 
    '  Milky Way  0 
    '  Andromeda  3 
End Sub 

Public Class Galaxy
    Public Property Name As String 
    Public Property MegaLightYears As Integer 
End Class

Many common collections are provided by the .NET Framework. Each type of collection is designed for a specific purpose.

The following groups of collection classes are described in this section:

  • System.Collections.Generic classes

  • System.Collections.Concurrent classes

  • System.Collections classes

  • Visual Basic Collection class

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(Of TKey, TValue)

Represents a collection of key/value pairs that are organized based on the key.

List(Of T)

Represents a list of objects that can be accessed by index. Provides methods to search, sort, and modify lists.

Queue(Of T)

Represents a first in, first out (FIFO) collection of objects.

SortedList(Of TKey, TValue)

Represents a collection of key/value pairs that are sorted by key based on the associated IComparer(Of T) implementation.

Stack(Of T)

Represents a last in, first out (LIFO) collection of objects.

For additional information, see Commonly Used Collection Types, Selecting a Collection Class, and System.Collections.Generic.

In the .NET Framework 4, 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(Of T), ConcurrentDictionary(Of TKey, TValue), ConcurrentQueue(Of T), and ConcurrentStack(Of T).

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.

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.

The Dictionary(Of 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.

Private Sub IterateThroughDictionary()
    Dim elements As Dictionary(Of String, Element) = BuildDictionary()

    For Each kvp As KeyValuePair(Of String, Element) In elements
        Dim theElement As Element = kvp.Value

        Console.WriteLine("key: " & kvp.Key)
        With theElement
            Console.WriteLine("values: " & .Symbol & " " &
                .Name & " " & .AtomicNumber)
        End With 
    Next 
End Sub 

Private Function BuildDictionary() As Dictionary(Of String, Element)
    Dim elements As New Dictionary(Of String, Element)

    AddToDictionary(elements, "K", "Potassium", 19)
    AddToDictionary(elements, "Ca", "Calcium", 20)
    AddToDictionary(elements, "Sc", "Scandium", 21)
    AddToDictionary(elements, "Ti", "Titanium", 22)

    Return elements
End Function 

Private Sub AddToDictionary(ByVal elements As Dictionary(Of String, Element),
ByVal symbol As String, ByVal name As String, ByVal atomicNumber As Integer)
    Dim theElement As New Element

    theElement.Symbol = symbol
    theElement.Name = name
    theElement.AtomicNumber = atomicNumber

    elements.Add(Key:=theElement.Symbol, value:=theElement)
End Sub 

Public Class Element
    Public Property Symbol As String 
    Public Property Name As String 
    Public Property AtomicNumber As Integer 
End Class

To instead use a collection initializer to build the Dictionary collection, you can replace the BuildDictionary and AddToDictionary methods with the following method.

Private Function BuildDictionary2() As Dictionary(Of String, Element)
    Return New Dictionary(Of String, Element) From
        {
            {"K", New Element With
                {.Symbol = "K", .Name = "Potassium", .AtomicNumber = 19}},
            {"Ca", New Element With
                {.Symbol = "Ca", .Name = "Calcium", .AtomicNumber = 20}},
            {"Sc", New Element With
                {.Symbol = "Sc", .Name = "Scandium", .AtomicNumber = 21}},
            {"Ti", New Element With
                {.Symbol = "Ti", .Name = "Titanium", .AtomicNumber = 22}}
        }
End Function

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, or elements[symbol] in C#.

Private Sub FindInDictionary(ByVal symbol As String)
    Dim elements As Dictionary(Of String, Element) = BuildDictionary()

    If elements.ContainsKey(symbol) = False Then
        Console.WriteLine(symbol & " not found")
    Else 
        Dim theElement = elements(symbol)
        Console.WriteLine("found: " & theElement.Name)
    End If 
End Sub

The following example instead uses the TryGetValue method quickly find an item by key.

Private Sub FindInDictionary2(ByVal symbol As String)
    Dim elements As Dictionary(Of String, Element) = BuildDictionary()

    Dim theElement As Element = Nothing 
    If elements.TryGetValue(symbol, theElement) = False Then
        Console.WriteLine(symbol & " not found")
    Else
        Console.WriteLine("found: " & theElement.Name)
    End If 
End Sub

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 or Getting Started with LINQ in C#.

The following example runs a LINQ query against a generic List. The LINQ query returns a different collection that contains the results.

Private Sub ShowLINQ()
    Dim elements As List(Of Element) = BuildList()

    ' LINQ Query. 
    Dim subset = From theElement In elements
                  Where theElement.AtomicNumber < 22
                  Order By theElement.Name

    For Each theElement In subset
        Console.WriteLine(theElement.Name & " " & theElement.AtomicNumber)
    Next 

    ' Output: 
    '  Calcium 20 
    '  Potassium 19 
    '  Scandium 21 
End Sub 

Private Function BuildList() As List(Of Element)
    Return New List(Of Element) From
        {
            {New Element With
                {.Symbol = "K", .Name = "Potassium", .AtomicNumber = 19}},
            {New Element With
                {.Symbol = "Ca", .Name = "Calcium", .AtomicNumber = 20}},
            {New Element With
                {.Symbol = "Sc", .Name = "Scandium", .AtomicNumber = 21}},
            {New Element With
                {.Symbol = "Ti", .Name = "Titanium", .AtomicNumber = 22}}
        }
End Function 

Public Class Element
    Public Property Symbol As String 
    Public Property Name As String 
    Public Property AtomicNumber As Integer 
End Class

The following example illustrates a procedure for sorting a collection. The example sorts instances of the Car class that are stored in a List(Of T). The Car class implements the IComparable(Of 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.

In the ListCars method, the cars.Sort() statement sorts the list. This call to the Sort method of the List(Of T) causes the CompareTo method to be called automatically for the Car objects in the List.

Public Sub ListCars()

    ' Create some new cars. 
    Dim cars As New List(Of Car) From
    {
        New Car With {.Name = "car1", .Color = "blue", .Speed = 20},
        New Car With {.Name = "car2", .Color = "red", .Speed = 50},
        New Car With {.Name = "car3", .Color = "green", .Speed = 10},
        New Car With {.Name = "car4", .Color = "blue", .Speed = 50},
        New Car With {.Name = "car5", .Color = "blue", .Speed = 30},
        New Car With {.Name = "car6", .Color = "red", .Speed = 60},
        New Car With {.Name = "car7", .Color = "green", .Speed = 50}
    }

    ' Sort the cars by color alphabetically, and then by speed 
    ' in descending order.
    cars.Sort()

    ' View all of the cars. 
    For Each thisCar As Car In cars
        Console.Write(thisCar.Color.PadRight(5) & " ")
        Console.Write(thisCar.Speed.ToString & " ")
        Console.Write(thisCar.Name)
        Console.WriteLine()
    Next 

    ' Output: 
    '  blue  50 car4 
    '  blue  30 car5 
    '  blue  20 car1 
    '  green 50 car7 
    '  green 10 car3 
    '  red   60 car6 
    '  red   50 car2 
End Sub 

Public Class Car
    Implements IComparable(Of Car)

    Public Property Name As String 
    Public Property Speed As Integer 
    Public Property Color As String 

    Public Function CompareTo(ByVal other As Car) As Integer _
        Implements System.IComparable(Of Car).CompareTo
        ' A call to this method makes a single comparison that is 
        ' used for sorting. 

        ' Determine the relative order of the objects being compared. 
        ' Sort by color alphabetically, and then by speed in 
        ' descending order. 

        ' Compare the colors. 
        Dim compare As Integer
        compare = String.Compare(Me.Color, other.Color, True)

        ' If the colors are the same, compare the speeds. 
        If compare = 0 Then
            compare = Me.Speed.CompareTo(other.Speed)

            ' Use descending order for speed.
            compare = -compare
        End If 

        Return compare
    End Function 
End Class

You can define a collection by implementing the IEnumerable(Of T) or IEnumerable interface. For additional information, see Enumerating a Collection and How to: Access a Collection Class with foreach (C# Programming Guide).

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.

Public Sub ListColors()
    Dim colors As New AllColors()

    For Each theColor As Color In colors
        Console.Write(theColor.Name & " ")
    Next
    Console.WriteLine()
    ' Output: red blue green 
End Sub 

' Collection class. 
Public Class AllColors
    Implements System.Collections.IEnumerable

    Private _colors() As Color =
    {
        New Color With {.Name = "red"},
        New Color With {.Name = "blue"},
        New Color With {.Name = "green"}
    }

    Public Function GetEnumerator() As System.Collections.IEnumerator _
        Implements System.Collections.IEnumerable.GetEnumerator

        Return New ColorEnumerator(_colors)

        ' Instead of creating a custom enumerator, you could 
        ' use the GetEnumerator of the array. 
        'Return _colors.GetEnumerator 
    End Function 

    ' Custom enumerator. 
    Private Class ColorEnumerator
        Implements System.Collections.IEnumerator

        Private _colors() As Color
        Private _position As Integer = -1

        Public Sub New(ByVal colors() As Color)
            _colors = colors
        End Sub 

        Public ReadOnly Property Current() As Object _
            Implements System.Collections.IEnumerator.Current
            Get 
                Return _colors(_position)
            End Get 
        End Property 

        Public Function MoveNext() As Boolean _
            Implements System.Collections.IEnumerator.MoveNext
            _position += 1
            Return (_position < _colors.Length)
        End Function 

        Public Sub Reset() Implements System.Collections.IEnumerator.Reset
            _position = -1
        End Sub 
    End Class 
End Class 

' Element class. 
Public Class Color
    Public Property Name As String 
End Class

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 (Visual Basic) or yield return (C#) statement to return each element of the collection one at a time.

You call an iterator by using a For Each…Next (Visual Basic) or foreach (C#) statement. Each iteration of the For Each loop calls the iterator. When a Yield or yield return 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 (C# and Visual Basic).

The following example uses an iterator method. The iterator method has a Yield or yield return statement that is inside a For…Next (Visual Basic) or for (C#) 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 or yield return statement.

Public Sub ListEvenNumbers()
    For Each number As Integer In EvenSequence(5, 18)
        Console.Write(number & " ")
    Next
    Console.WriteLine()
    ' Output: 6 8 10 12 14 16 18 
End Sub 

Private Iterator Function EvenSequence(
ByVal firstNumber As Integer, ByVal lastNumber As Integer) _
As IEnumerable(Of Integer)

' Yield even numbers in the range. 
    For number = firstNumber To lastNumber
        If number Mod 2 = 0 Then
            Yield number
        End If 
    Next 
End Function
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