IDictionary.Item[Object] Property

Definition

Gets or sets the element with the specified key.

C#
public object this[object key] { get; set; }
C#
public object? this[object key] { get; set; }

Parameters

key
Object

The key of the element to get or set.

Property Value

The element with the specified key, or null if the key does not exist.

Exceptions

key is null.

The property is set and the IDictionary object is read-only.

-or-

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

Examples

The following code example demonstrates how to implement the Item[] property. This code example is part of a larger example provided for the IDictionary class.

C#
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);
        }
    }
}

Remarks

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 dictionary (for example, myCollection["myNonexistentKey"] = myValue). However, if the specified key already exists in the dictionary, setting the Item[] property overwrites the old value. In contrast, the Add method does not modify existing elements.

Implementations can vary in whether they allow the key to be null.

The C# language uses the thisthis keyword to define the indexers instead of implementing the Item[] property. Visual Basic implements Item[] as a default property, which provides the same indexing functionality.

Applies to

Product Versions
.NET Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9
.NET Framework 1.1, 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1
.NET Standard 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 2.0, 2.1
UWP 10.0

See also