How Can I Efficiently Retrieve Multiple Keys Associated with a Single Value in a .NET Dictionary?

.NET Generic Dictionary: Handling Multiple Keys for a Single Value

Standard .NET Dictionary objects offer easy key-to-value access. However, retrieving keys associated with a specific value becomes complex when multiple keys map to the same value. This article presents a solution using a custom bidirectional dictionary.

Building a Bidirectional Dictionary

To overcome this limitation, we create a BiDictionary<TFirst, TSecond> class. This class manages key-value relationships in both directions, using two internal dictionaries: firstToSecond (key to value) and secondToFirst (value to keys).

Adding Entries and Accessing Data

The Add method efficiently handles adding new key-value pairs. It checks for existing entries and creates new lists if necessary to store multiple keys for a single value.

The BiDictionary class provides indexers for both key-to-value and value-to-key access. To avoid ambiguity, dedicated methods, GetByFirst and GetBySecond, are also included.

Illustrative Example

Let's consider a practical example:

<code class="language-C#">BiDictionary<int, string> greek = new BiDictionary<int, string>();
greek.Add(1, "Alpha");
greek.Add(2, "Beta");
greek.Add(5, "Beta");

ShowEntries(greek, "Alpha"); // Output: Alpha: [1]
ShowEntries(greek, "Beta"); // Output: Beta: [2, 5]
ShowEntries(greek, "Gamma"); // Output: Gamma: []</code>

The ShowEntries method demonstrates retrieving keys for a given value. Notice how "Beta" correctly returns both associated keys (2 and 5). This showcases the bidirectional dictionary's ability to handle multiple keys per value.

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