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Collections (Visual Basic)

2015-07-20

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Collections (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.

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 xref:System.Collections.Generic?displayProperty=nameWithType 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.

In this topic

Using a Simple Collection
Kinds of Collections
Implementing a Collection of Key/Value Pairs
Using LINQ to Access a Collection
Sorting a Collection
Defining a Custom Collection
Iterators

Using a Simple Collection

The examples in this section use the generic xref:System.Collections.Generic.List%601 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.

' 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.

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 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 statement that iterates in descending order is used. This is because the xref:System.Collections.Generic.List%601.RemoveAt%2A 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 xref:System.Collections.Generic.List%601, you can also define your own class. In the following example, the Galaxy class that is used by the xref:System.Collections.Generic.List%601 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

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 xref: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 xref:System.Collections.Generic?displayProperty=nameWithType namespace:

Class	Description
xref:System.Collections.Generic.Dictionary%602	Represents a collection of key/value pairs that are organized based on the key.
xref:System.Collections.Generic.List%601	Represents a list of objects that can be accessed by index. Provides methods to search, sort, and modify lists.
xref:System.Collections.Generic.Queue%601	Represents a first in, first out (FIFO) collection of objects.
xref:System.Collections.Generic.SortedList%602	Represents a collection of key/value pairs that are sorted by key based on the associated xref:System.Collections.Generic.IComparer%601 implementation.
xref:System.Collections.Generic.Stack%601	Represents a last in, first out (LIFO) collection of objects.

For additional information, see Commonly Used Collection Types, Selecting a Collection Class, and xref:System.Collections.Generic?displayProperty=nameWithType.

System.Collections.Concurrent Classes

In the .NET Framework 4 or newer, the collections in the xref:System.Collections.Concurrent namespace provide efficient thread-safe operations for accessing collection items from multiple threads.

The classes in the xref:System.Collections.Concurrent namespace should be used instead of the corresponding types in the xref:System.Collections.Generic?displayProperty=nameWithType and xref:System.Collections?displayProperty=nameWithType namespaces whenever multiple threads are accessing the collection concurrently. For more information, see Thread-Safe Collections and xref:System.Collections.Concurrent.

Some classes included in the xref:System.Collections.Concurrent namespace are xref:System.Collections.Concurrent.BlockingCollection%601, xref:System.Collections.Concurrent.ConcurrentDictionary%602, xref:System.Collections.Concurrent.ConcurrentQueue%601, and xref:System.Collections.Concurrent.ConcurrentStack%601.

System.Collections Classes

The classes in the xref:System.Collections?displayProperty=nameWithType 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 xref:System.Collections.Generic?displayProperty=nameWithType namespace or the xref: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
xref:System.Collections.ArrayList	Represents an array of objects whose size is dynamically increased as required.
xref:System.Collections.Hashtable	Represents a collection of key/value pairs that are organized based on the hash code of the key.
xref:System.Collections.Queue	Represents a first in, first out (FIFO) collection of objects.
xref:System.Collections.Stack	Represents a last in, first out (LIFO) collection of objects.

The xref: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 xref:Microsoft.VisualBasic.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.

For more information, see xref:Microsoft.VisualBasic.Collection.

Implementing a Collection of Key/Value Pairs

The xref:System.Collections.Generic.Dictionary%602 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 xref:System.Collections.Generic.Dictionary%602.ContainsKey%2A method and the xref:System.Collections.Generic.Dictionary%602.Item%2A 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.

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 xref:System.Collections.Generic.Dictionary%602.TryGetValue%2A 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

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.

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

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 xref:System.Collections.Generic.List%601. The Car class implements the xref:System.IComparable%601 interface, which requires that the xref:System.IComparable%601.CompareTo%2A method be implemented.

Each call to the xref:System.IComparable%601.CompareTo%2A 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 xref:System.Collections.Generic.List%601.Sort%2A method of the xref:System.Collections.Generic.List%601 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

Defining a Custom Collection

You can define a collection by implementing the xref:System.Collections.Generic.IEnumerable%601 or xref:System.Collections.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 xref:System.Collections.IEnumerable interface, which requires that the xref:System.Collections.IEnumerable.GetEnumerator%2A method be implemented.

The GetEnumerator method returns an instance of the ColorEnumerator class. ColorEnumerator implements the xref:System.Collections.IEnumerator interface, which requires that the xref:System.Collections.IEnumerator.Current%2A property, xref:System.Collections.IEnumerator.MoveNext%2A method, and xref:System.Collections.IEnumerator.Reset%2A 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

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.

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

dotnet/docs

Users who have contributed to this file

Collections (Visual Basic)

Using a Simple Collection

Kinds of Collections

System.Collections.Generic Classes

System.Collections.Concurrent Classes

System.Collections Classes

Visual Basic Collection Class

Implementing a Collection of Key/Value Pairs

Using LINQ to Access a Collection

Sorting a Collection

Defining a Custom Collection

Iterators

See Also