PropertyCollection.IDictionary.Item[Object] 属性

定义

获取或设置具有指定键的元素。

property System::Object ^ System::Collections::IDictionary::Item[System::Object ^] { System::Object ^ get(System::Object ^ key); void set(System::Object ^ key, System::Object ^ value); };
object System.Collections.IDictionary.Item[object key] { get; set; }
object? System.Collections.IDictionary.Item[object key] { get; set; }
member this.System.Collections.IDictionary.Item(obj) : obj with get, set
 Property Item(key As Object) As Object Implements IDictionary.Item

参数

key
Object

要获取或设置的元素的键。

属性值

带有指定键的元素。

实现

例外

keynull

设置该属性,而且 IDictionary 对象为只读。

- 或 -

设置了该属性,但 key 不存在于集合中,且 IDictionary 具有固定的大小。

示例

以下示例演示如何实现 Item[] 属性。 此代码示例是为 IDictionary 类提供的一个更大示例的一部分。

public:
    virtual property Object^ default[Object^]
    {
        Object^ get(Object^ key)
        {
            // If this key is in the dictionary, return its value.
            int index;
            if (TryGetIndexOfKey(key, &index))
            {
                // The key was found; return its value.
                return items[index]->Value;
            }
            else
            {
                // The key was not found; return null.
                return nullptr;
            }
        }

        void set(Object^ key, Object^ value)
        {
            // If this key is in the dictionary, change its value.
            int index;
            if (TryGetIndexOfKey(key, &index))
            {
                // The key was found; change its value.
                items[index]->Value = value;
            }
            else
            {
                // This key is not in the dictionary; add this
                // key/value pair.
                Add(key, value);
            }
        }
    }
public object this[object key]
{
    get
    {
        // If this key is in the dictionary, return its value.
        Int32 index;
        if (TryGetIndexOfKey(key, out index))
        {
            // The key was found; return its value.
            return items[index].Value;
        }
        else
        {
            // The key was not found; return null.
            return null;
        }
    }

    set
    {
        // If this key is in the dictionary, change its value.
        Int32 index;
        if (TryGetIndexOfKey(key, out index))
        {
            // The key was found; change its value.
            items[index].Value = value;
        }
        else
        {
            // This key is not in the dictionary; add this key/value pair.
            Add(key, value);
        }
    }
}
Public Property Item(ByVal key As Object) As Object Implements IDictionary.Item
    Get

        ' If this key is in the dictionary, return its value.
        Dim index As Integer
        If TryGetIndexOfKey(key, index) Then

            ' The key was found return its value.
            Return items(index).Value
        Else

            ' The key was not found return null.
            Return Nothing
        End If
    End Get

    Set(ByVal value As Object)
        ' If this key is in the dictionary, change its value. 
        Dim index As Integer
        If TryGetIndexOfKey(key, index) Then

            ' The key was found change its value.
            items(index).Value = value
        Else

            ' This key is not in the dictionary add this key/value pair.
            Add(key, value)
        End If
    End Set
End Property

注解

通过此属性,可以使用以下语法来访问集合中的特定元素:myCollection[key]

还可以使用 Item[] 属性通过设置字典中不存在的键的值来添加新元素 (例如 myCollection["myNonexistentKey"] = myValue ,) 。 但是,如果字典中已存在指定的键,则设置 Item[] 属性将覆盖旧值。 相反, Add 方法不修改现有元素。

适用于

另请参阅