


.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></tfirst>
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:
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: []
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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