How to: Create a Collection of Objects 

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As with any object, you declare a variable to hold the object, then create the collection object and assign it to the variable.

For a collection object, you can use either the Visual Basic Collection Class or a .NET Framework collection class. In particular, 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. Generic collections enforce strong typing by allowing only the desired data type to be added. For more information, see How to: Define Type-Safe Collections.

Once the collection object is created, you can add and remove items and access items in the collection.

Two examples about creating collections follow. Each collection holds String items and associates a String key with each item. The first two procedures create a collection using the Visual Basic collection class. The last two procedures create a collection using a .NET Framework generic collection class.

To create a collection using the Visual Basic collection class

  1. Declare and create a Visual Basic Collection variable, as the following example shows.

    Dim sampleVisualBasicColl As New Microsoft.VisualBasic.Collection()

    The collection in sampleVisualBasicColl can accept items of any data type.

  2. Use the Add Method (Collection Object) to add elements to the collection. The following example creates four String elements and adds them to the collection. It creates a unique String value as the key for each new element and passes this value to the Add method.

    Dim item1, item2, item3, item4 As String
item1 = "Items"
item2 = "In"
item3 = "A"
item4 = "Collection"
sampleVisualBasicColl.Add(item1, "firstkey")
sampleVisualBasicColl.Add(item2, "secondkey")
sampleVisualBasicColl.Add(item3, "thirdkey")
sampleVisualBasicColl.Add(item4, "fourthkey")

    The Key argument is optional in a Visual Basic collection.

  3. If you want to remove an element from the collection, you can use the Remove Method (Collection Object), identifying the element either by its positional index or by its optional key. The following example illustrates this.

    ' Remove the first element of the Visual Basic collection.
sampleVisualBasicColl.Remove(1)
' Remove the element with the key "secondkey".
sampleVisualBasicColl.Remove("secondkey")

    Note that when an element is removed from a Visual Basic Collection, the index values are renumbered from 1 through the value of the Count Property (Collection Object).

To use For Each...Next to process the elements of your Visual Basic collection

  1. Declare a variable of the type stored in the collection. For the preceding example, declare a variable of type String, as the following example shows.

    ' Insert code from the preceding example.
Dim aString As String

  2. Use a For Each...Next Statement (Visual Basic) to examine each element of the collection. The following example searches for a particular string and displays it if found.

    For Each aString in sampleVisualBasicColl
    If aString = "Collection" Then
        MsgBox(aString)
    End If
Next aString

To create a collection using a generic collection class

  1. Declare and create a .NET Framework System.Collections.Generic.Dictionary variable, as the following example shows.

    Dim sampleGenericColl As New System.Collections.Generic.Dictionary(Of String, String)

    The sampleGenericColl variable holds a type-safe collection that accepts items and keys only of type String.

  2. Use the System.Collections.Generic.Dictionary.Add( method to add elements to the collection. The following example creates four String elements and adds them to the collection. It creates a unique String value as the key for each new element and passes this value to the Add method.

    Dim item1, item2, item3, item4 As String
item1 = "Items"
item2 = "In"
item3 = "A"
item4 = "Collection"
sampleGenericColl.Add("firstkey", item1)
sampleGenericColl.Add("secondkey", item2)
sampleGenericColl.Add("thirdkey", item3)
sampleGenericColl.Add("fourthkey", item4)

    The Key argument is required in this generic collection.

  3. To remove an element from the collection, use the System.Collections.Generic.IDictionary.Remove( method. You must supply the key to identify the element to remove. The following example illustrates this.

    If Not sampleGenericColl.Remove("thirdkey")
    ' Insert code to handle "thirdkey" not found in collection.
End If

    You can use a For Each...Next statement to loop through and process the elements of a collection, as the following procedure demonstrates.

To use For Each...Next to process the elements of your generic collection

  1. Declare a variable of the type stored in the collection. For the preceding example, declare a variable of type String, as the following example shows.

    ' Insert code from the preceding example.
Dim aPair As KeyValuePair(Of String, String)

  2. Use a For Each...Next Statement (Visual Basic) to examine each element of the collection. The following example searches for a particular string and displays it if found.

    For Each aPair In sampleGenericColl
    If aPair.Value = "Items" Then
        MsgBox(aPair.Key & " -- " & aPair.Value)
    End If
Next aPair

See Also

Tasks

How to: Create an Array of Objects

Reference

Collection Object (Visual Basic)
System.Collections
System.Collections.Generic
System.Collections.Specialized

Concepts

Visual Basic Collection Class