DictionaryBase::IDictionary::Item Property
Gets or sets the value associated with the specified key.
Assembly: mscorlib (in mscorlib.dll)
private: virtual property Object^ Item[Object^ key] { Object^ get (Object^ key) sealed = IDictionary::Item::get; void set (Object^ key, Object^ value) sealed = IDictionary::Item::set; }
Parameters
- key
- Type: System::Object
The key whose value to get or set.
Property Value
Type: System::ObjectThe value associated with the specified key. If the specified key is not found, attempting to get it returns nullptr, and attempting to set it creates a new element using the specified key.
Implements
IDictionary::Item[Object]| Exception | Condition |
|---|---|
| ArgumentNullException | key is nullptr. |
| 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.
using namespace System; using namespace System::Collections; public ref class ShortStringDictionary: public DictionaryBase { public: property String^ Item [String^] { String^ get( String^ key ) { return (dynamic_cast<String^>(Dictionary[ key ])); } void set( String^ value, String^ key ) { Dictionary[ key ] = value; } } property ICollection^ Keys { ICollection^ get() { return (Dictionary->Keys); } } property ICollection^ Values { ICollection^ get() { return (Dictionary->Values); } } void Add( String^ key, String^ value ) { Dictionary->Add( key, value ); } bool Contains( String^ key ) { return (Dictionary->Contains( key )); } void Remove( String^ key ) { Dictionary->Remove( key ); } protected: virtual void OnInsert( Object^ key, Object^ value ) override { if ( key->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "key must be of type String.","key" ); else { String^ strKey = dynamic_cast<String^>(key); if ( strKey->Length > 5 ) throw gcnew ArgumentException( "key must be no more than 5 characters in length.","key" ); } if ( value->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "value must be of type String.","value" ); else { String^ strValue = dynamic_cast<String^>(value); if ( strValue->Length > 5 ) throw gcnew ArgumentException( "value must be no more than 5 characters in length.","value" ); } } virtual void OnRemove( Object^ key, Object^ /*value*/ ) override { if ( key->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "key must be of type String.","key" ); else { String^ strKey = dynamic_cast<String^>(key); if ( strKey->Length > 5 ) throw gcnew ArgumentException( "key must be no more than 5 characters in length.","key" ); } } virtual void OnSet( Object^ key, Object^ /*oldValue*/, Object^ newValue ) override { if ( key->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "key must be of type String.","key" ); else { String^ strKey = dynamic_cast<String^>(key); if ( strKey->Length > 5 ) throw gcnew ArgumentException( "key must be no more than 5 characters in length.","key" ); } if ( newValue->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "newValue must be of type String.","newValue" ); else { String^ strValue = dynamic_cast<String^>(newValue); if ( strValue->Length > 5 ) throw gcnew ArgumentException( "newValue must be no more than 5 characters in length.","newValue" ); } } virtual void OnValidate( Object^ key, Object^ value ) override { if ( key->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "key must be of type String.","key" ); else { String^ strKey = dynamic_cast<String^>(key); if ( strKey->Length > 5 ) throw gcnew ArgumentException( "key must be no more than 5 characters in length.","key" ); } if ( value->GetType() != Type::GetType( "System.String" ) ) throw gcnew ArgumentException( "value must be of type String.","value" ); else { String^ strValue = dynamic_cast<String^>(value); if ( strValue->Length > 5 ) throw gcnew ArgumentException( "value must be no more than 5 characters in length.","value" ); } } }; void PrintKeysAndValues2( ShortStringDictionary^ myCol ); void PrintKeysAndValues3( ShortStringDictionary^ myCol ); int main() { // Creates and initializes a new DictionaryBase. ShortStringDictionary^ mySSC = gcnew 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 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 ( ArgumentException^ e ) { Console::WriteLine( e ); } // Tries to add a key that is too long. try { mySSC->Add( "Eleven", "ijk" ); } catch ( ArgumentException^ e ) { Console::WriteLine( e ); } 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\":" ); PrintKeysAndValues2( mySSC ); } // Uses the enumerator. void PrintKeysAndValues2( ShortStringDictionary^ myCol ) { DictionaryEntry myDE; System::Collections::IEnumerator^ myEnumerator = myCol->GetEnumerator(); while ( myEnumerator->MoveNext() ) if ( myEnumerator->Current != nullptr ) { myDE = *dynamic_cast<DictionaryEntry^>(myEnumerator->Current); Console::WriteLine( " {0,-5} : {1}", myDE.Key, myDE.Value ); } Console::WriteLine(); } // Uses the Keys property and the Item property. void PrintKeysAndValues3( ShortStringDictionary^ myCol ) { ICollection^ myKeys = myCol->Keys; IEnumerator^ myEnum1 = myKeys->GetEnumerator(); while ( myEnum1->MoveNext() ) { String^ k = safe_cast<String^>(myEnum1->Current); Console::WriteLine( " {0,-5} : {1}", k, myCol->Item[ k ] ); } Console::WriteLine(); } /* This code produces the following output. 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 */
Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2
The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.