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DictionaryBase.IDictionary.Item Property

Gets or sets the value associated with the specified key.

Namespace:  System.Collections
Assembly:  mscorlib (in mscorlib.dll)

'Declaration
Private Property Item ( _
	key As Object _
) As Object Implements IDictionary.Item

Parameters

key
Type: System.Object

The key whose value to get or set.

Property Value

Type: System.Object
The value associated with the specified key. If the specified key is not found, attempting to get it returns Nothing, and attempting to set it creates a new element using the specified key.

Implements

IDictionary.Item

ExceptionCondition
ArgumentNullException

key is Nothing.

NotSupportedException

The property is set and the DictionaryBase is read-only.

-or-

The property is set, key does not exist in the collection, and the DictionaryBase has a fixed size.

This property provides the ability to access a specific element in the collection by using the following syntax: myCollection[key].

You can also use the Item property to add new elements by setting the value of a key that does not exist in the DictionaryBase; for example, myCollection["myNonexistentKey"] = myValue. However, if the specified key already exists in the DictionaryBase, setting the Item property overwrites the old value. In contrast, the Add method does not modify existing elements.

Retrieving the value of this property is an O(1) operation; setting the property is also an O(1) operation.

The following code example implements the DictionaryBase class and uses that implementation to create a dictionary of String keys and values that have a Length of 5 characters or less.

Imports System
Imports System.Collections

Public Class ShortStringDictionary
   Inherits DictionaryBase

   Default Public Property Item(key As String) As String 
      Get 
         Return CType(Dictionary(key), String)
      End Get 
      Set
         Dictionary(key) = value
      End Set 
   End Property 

   Public ReadOnly Property Keys() As ICollection
      Get 
         Return Dictionary.Keys
      End Get 
   End Property 

   Public ReadOnly Property Values() As ICollection
      Get 
         Return Dictionary.Values
      End Get 
   End Property 

   Public Sub Add(key As String, value As String)
      Dictionary.Add(key, value)
   End Sub 'Add

   Public Function Contains(key As String) As Boolean 
      Return Dictionary.Contains(key)
   End Function 'Contains

   Public Sub Remove(key As String)
      Dictionary.Remove(key)
   End Sub 'Remove

   Protected Overrides Sub OnInsert(key As Object, value As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If  
      If Not GetType(System.String).IsAssignableFrom(value.GetType()) Then 
         Throw New ArgumentException("value must be of type String.", "value")
      Else 
         Dim strValue As String = CType(value, String)
         If strValue.Length > 5 Then 
            Throw New ArgumentException("value must be no more than 5 characters in length.", "value")
         End If 
      End If 
   End Sub 'OnInsert

   Protected Overrides Sub OnRemove(key As Object, value As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If 
   End Sub 'OnRemove

   Protected Overrides Sub OnSet(key As Object, oldValue As Object, newValue As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If  
      If Not GetType(System.String).IsAssignableFrom(newValue.GetType()) Then 
         Throw New ArgumentException("newValue must be of type String.", "newValue")
      Else 
         Dim strValue As String = CType(newValue, String)
         If strValue.Length > 5 Then 
            Throw New ArgumentException("newValue must be no more than 5 characters in length.", "newValue")
         End If 
      End If 
   End Sub 'OnSet

   Protected Overrides Sub OnValidate(key As Object, value As Object)
      If Not GetType(System.String).IsAssignableFrom(key.GetType()) Then 
         Throw New ArgumentException("key must be of type String.", "key")
      Else 
         Dim strKey As String = CType(key, String)
         If strKey.Length > 5 Then 
            Throw New ArgumentException("key must be no more than 5 characters in length.", "key")
         End If 
      End If  
      If Not GetType(System.String).IsAssignableFrom(value.GetType()) Then 
         Throw New ArgumentException("value must be of type String.", "value")
      Else 
         Dim strValue As String = CType(value, String)
         If strValue.Length > 5 Then 
            Throw New ArgumentException("value must be no more than 5 characters in length.", "value")
         End If 
      End If 
   End Sub 'OnValidate 

End Class 'ShortStringDictionary


Public Class SamplesDictionaryBase

   Public Shared Sub Main()

      ' Creates and initializes a new DictionaryBase. 
      Dim mySSC As New ShortStringDictionary()

      ' Adds elements to the collection.
      mySSC.Add("One", "a")
      mySSC.Add("Two", "ab")
      mySSC.Add("Three", "abc")
      mySSC.Add("Four", "abcd")
      mySSC.Add("Five", "abcde")

      ' Display the contents of the collection using For Each. This is the preferred method.
      Console.WriteLine("Contents of the collection (using For Each):")
      PrintKeysAndValues1(mySSC)

      ' Display the contents of the collection using the enumerator.
      Console.WriteLine("Contents of the collection (using enumerator):")
      PrintKeysAndValues2(mySSC)

      ' Display the contents of the collection using the Keys property and the Item property.
      Console.WriteLine("Initial contents of the collection (using Keys and Item):")
      PrintKeysAndValues3(mySSC)

      ' Tries to add a value that is too long. 
      Try
          mySSC.Add("Ten", "abcdefghij")
      Catch e As ArgumentException
          Console.WriteLine(e.ToString())
      End Try 

      ' Tries to add a key that is too long. 
      Try
          mySSC.Add("Eleven", "ijk")
      Catch e As ArgumentException
          Console.WriteLine(e.ToString())
      End Try

      Console.WriteLine()

      ' Searches the collection with Contains.
      Console.WriteLine("Contains ""Three"": {0}", mySSC.Contains("Three"))
      Console.WriteLine("Contains ""Twelve"": {0}", mySSC.Contains("Twelve"))
      Console.WriteLine()

      ' Removes an element from the collection.
      mySSC.Remove("Two")

      ' Displays the contents of the collection.
      Console.WriteLine("After removing ""Two"":")
      PrintKeysAndValues1(mySSC)

    End Sub 'Main


    ' Uses the For Each statement which hides the complexity of the enumerator. 
    ' NOTE: The For Each statement is the preferred way of enumerating the contents of a collection. 
    Public Shared Sub PrintKeysAndValues1(myCol As ShortStringDictionary)
      Dim myDE As DictionaryEntry
      For Each myDE In  myCol
          Console.WriteLine("   {0,-5} : {1}", myDE.Key, myDE.Value)
      Next myDE
      Console.WriteLine()
    End Sub 'PrintKeysAndValues1


    ' Uses the enumerator.  
    ' NOTE: The For Each statement is the preferred way of enumerating the contents of a collection. 
    Public Shared Sub PrintKeysAndValues2(myCol As ShortStringDictionary)
      Dim myDE As DictionaryEntry
      Dim myEnumerator As System.Collections.IEnumerator = myCol.GetEnumerator()
      While myEnumerator.MoveNext()
          If Not (myEnumerator.Current Is Nothing) Then
            myDE = CType(myEnumerator.Current, DictionaryEntry)
            Console.WriteLine("   {0,-5} : {1}", myDE.Key, myDE.Value)
          End If 
      End While
      Console.WriteLine()
    End Sub 'PrintKeysAndValues2


    ' Uses the Keys property and the Item property. 
    Public Shared Sub PrintKeysAndValues3(myCol As ShortStringDictionary)
      Dim myKeys As ICollection = myCol.Keys
      Dim k As String 
      For Each k In  myKeys
          Console.WriteLine("   {0,-5} : {1}", k, myCol(k))
      Next k
      Console.WriteLine()
    End Sub 'PrintKeysAndValues3

End Class 'SamplesDictionaryBase 


'This code produces the following output. 

'Contents of the collection (using For Each): 
'   Three : abc 
'   Five  : abcde 
'   Two   : ab 
'   One   : a 
'   Four  : abcd 

'Contents of the collection (using enumerator): 
'   Three : abc 
'   Five  : abcde 
'   Two   : ab 
'   One   : a 
'   Four  : abcd 

'Initial contents of the collection (using Keys and Item): 
'   Three : abc 
'   Five  : abcde 
'   Two   : ab 
'   One   : a 
'   Four  : abcd 

'System.ArgumentException: value must be no more than 5 characters in length. 
'Parameter name: value 
'   at ShortStringDictionary.OnValidate(Object key, Object value) 
'   at System.Collections.DictionaryBase.System.Collections.IDictionary.Add(Object key, Object value) 
'   at SamplesDictionaryBase.Main() 
'System.ArgumentException: key must be no more than 5 characters in length. 
'Parameter name: key 
'   at ShortStringDictionary.OnValidate(Object key, Object value) 
'   at System.Collections.DictionaryBase.System.Collections.IDictionary.Add(Object key, Object value) 
'   at SamplesDictionaryBase.Main() 

'Contains "Three": True 
'Contains "Twelve": False 

'After removing "Two": 
'   Three : abc 
'   Five  : abcde 
'   One   : a 
'   Four  : abcd

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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