Indexers

Recall that the System.Collections.Generic.Dictionary<TKey, TValue> class (see “The Dictionary<TKey, TValue> Class”) allows keys to be used as indices for the purpose of adding new keys and values, changing the value associated with a key, and retrieving the value associated with a key in the table. In this section, we will discuss how to implement this functionality.

An indexer in C# is defined using the following syntax:

public TValue this[TKey k]
{
    get
    {
        // Code to retrieve the value with key k
    }
    set
    {
        // Code to associate the given value with key k
    }
}

Note the resemblance of the above code to the definition of a property. The biggest differences are:

• In place of a property name, an indexer uses the keyword this.
• The keyword this is followed by a nonempty parameter list enclosed in square brackets.

Thus, an indexer is like a property with parameters. The parameters are the indices themselves; i.e., if d is a Dictionary<TKey, TValue> and key is a TKey, d[key] invokes the indexer with parameter key. In general, either the get accessor or the set accessor may be omitted, but at least one of them must be included. As in a property definition, the set accessor can use the keyword value for the value being assigned - in this case, the value to be associated with the given key. The value keyword and the return type of the get accessor will both be of type TValue, the type given prior to the keyword this in the above code.

We want to implement the indexer to behave in the same way as the indexer for System.Collections.Generic.Dictionary<TKey, TValue>. Thus, the get accessor is similar to the TryGetValue method, as outlined in “A Simple Hash Table Implementation”, with a few important differences. First, the get accessor has no out parameter. Instead, it returns the value that TryGetValue assigns to its out parameter when the key is found. When the key is not found, because it can’t return a bool to indicate this, it instead throws a KeyNotFoundException.

Likewise, the set accessor is similar to the Add method, as outlined in “A Simple Hash Table Implementation”. However, whereas the Add method has a TValue parameter through which to pass the value to be associated with the given key, the set accessor gets this value from the value keyword. Furthermore, we don’t want the set accessor to throw an exception if the key is found. Instead, we want it to replace the Data of the cell containing this key with a new KeyValuePair containing the key with the new value.