DotNetXtensions.BidirectionalDictionary

BidirectionalDictionary

A high-performance bidirectional dictionary implementation for C# that maintains one-to-one relationships between keys and values, allowing O(1) lookups in both directions. A carefully thought out implementation, amongst which is a configurable setter behavior via the Force property: controls whether setting a key's value silently evicts any conflicting reverse mapping (force mode, the default, ie the more lenient), or throws when value is already owned by a different key, aligned with important other BiMap libraries, like Guava's (Java based) BiMap. An AllowDefaults property (default: true) optionally rejects keys or values that equal their type's default — 0 for int, Guid.Empty for Guid, etc.

Acknowledgments

This library is a fork of ashishkarn's excellent TwoWayDictionary — "a robust, reusable bidirectional mapping solution for the C# community". Key modifications include: configurable setter behavior via Force/Set(key, value, force), an optimized Set implementation, the reverse indexer this[TValue], custom comparer support, AllowDefaults guard, implements IDictionary<TKey, TValue> now, etc.

Features

• Bidirectional Lookups: Retrieve values by keys or keys by values with O(1) time complexity
• One-to-One Mapping: Enforces unique keys and unique values - each key maps to exactly one value and vice versa
• Type-Safe: Generic implementation with compile-time type checking
• Configurable Setter Behavior: The Force property controls whether conflicting reverse mappings are silently evicted or cause an exception
• Default Value Guard: The AllowDefaults property (default: true) can be set to false to reject keys or values equal to their type's default (0, Guid.Empty, false, etc.)
• Comprehensive API: Support for Add, Set, Remove, Contains, and Try* variants
• IEnumerable Support: Iterate through key-value pairs with foreach
• Well-Tested: Comprehensive unit test coverage, 100% passing
• Zero Dependencies: No external dependencies beyond .NET 8.0+
• Multi-Target: Supports .NET 8.0 and .NET 10.0

Installation

Install via NuGet Package Manager:

dotnet add package DotNetXtensions.BidirectionalDictionary

Or via Package Manager Console:

Install-Package DotNetXtensions.BidirectionalDictionary

Quick Start

using DotNetXtensions;

// Create a two-way dictionary
BidirectionalDictionary<int, string> userMap = [];

// Add mappings
userMap.Add(101, "Alice");
userMap.Add(102, "Bob");
userMap.Add(103, "Charlie");

// Forward lookup (key -> value)
string name = userMap[101];  // "Alice"

// Reverse lookup (value -> key)
int id = userMap["Bob"];         // 102 (reverse indexer)
int id2 = userMap.GetKey("Bob"); // 102 (explicit method)

// Check existence
bool hasUser = userMap.ContainsKey(101);      // true
bool hasName = userMap.ContainsValue("Bob");  // true

// Remove mappings
userMap.RemoveByKey(102);
// or
userMap.RemoveByValue("Charlie");

API Documentation

Adding Elements

// Add - throws if key or value already exists
map.Add(key, value);

// TryAdd - returns false if key or value already exists
bool success = map.TryAdd(key, value);

// Set - updates mapping, behavior on value conflict controlled by Force property
map.Set(key, value);

// Set with explicit force override (ignores Force property for this call)
map.Set(key, value, force: true);   // forcePut: silently evict conflicting mapping
map.Set(key, value, force: false);  // strict put: throw if value owned by different key

// Indexer set - same as Set (uses Force property)
map[key] = value;

// TrySet - returns false (without throwing, without modifying the dictionary) if value is
// owned by a different key; on false, differentKeyOwns identifies the conflicting key
bool set = map.TrySet(key, value, out TKey? differentKeyOwns);

Setter Behavior: the Force Property

The Force property (default: true) controls what happens when you set a key's value and the new value is already mapped to a different key:

var map = new BidirectionalDictionary<int, string>();
map.Add(1, "Alice");
map.Add(2, "Bob");

// Force = true (default): silently evicts the conflicting mapping (1→"Alice" removed)
map.Force = true;
map[2] = "Alice";  // map now contains only: 2→"Alice"

// Force = false: throws ArgumentException instead
map.Force = false;
map[2] = "Alice";  // throws: "Value 'Alice' is already mapped to key '1'"

This maps to Guava's BiMap terminology: Force = true behaves like forcePut, Force = false behaves like put. The Set(key, value, bool force) overload lets you override this per-call regardless of the property.

Default Value Guard: the AllowDefaults Property

The AllowDefaults property (default: true) controls whether keys or values equal to their type's default are permitted. Set it to false to enforce that all entries carry meaningful, non-default values — useful when 0, Guid.Empty, false, and similar sentinel-like defaults would be ambiguous or invalid in your domain. On first look, this may not seem like a big deal, but when you consider, with a 1 to 1 restriction like this type has, it makes sense to never allow it. We still leave that in your hands, and thus have even kept this to true by default, but the point is, this is actually a significant issue.

// AllowDefaults = true (default): 0, Guid.Empty, false, etc. are accepted
var map = new BidirectionalDictionary<string, int>();
map.Add("pending", 0);  // fine

// AllowDefaults = false: throws ArgumentException for any default key or value
var strictMap = new BidirectionalDictionary<string, int> { AllowDefaults = false };
strictMap.Add("user", 0);    // throws: value 0 is the default for int
strictMap.Add("user", 42);   // fine

// Also guards keys
var idMap = new BidirectionalDictionary<int, string> { AllowDefaults = false };
idMap.Add(0, "alice");   // throws: key 0 is the default for int
idMap.Add(42, "alice");  // fine

// Works for any value type
var guidMap = new BidirectionalDictionary<string, Guid> { AllowDefaults = false };
guidMap.Add("x", Guid.Empty);   // throws
guidMap.Add("x", Guid.NewGuid()); // fine

The guard applies to any adds or sets (Add, TryAdd, and Set, indexer setter, etc). AllowDefaults and Force are independent — both can be set freely.

TrySet: Conflict-Aware Setting

TrySet is a third option alongside Force = true and Force = false. Rather than silently evicting a conflicting mapping or throwing an exception, it returns false and leaves the dictionary completely untouched, letting the caller decide what to do next.

This is a very useful feature, because the Set already had to do this check anyways. It allows you to have !force functionality (not silent overwrite of a conflict), but to allow the dictionary itself to do the work of checking if there will be a conflict.

The out TKey? differentKeyOwns parameter tells you exactly which key already owns the conflicting value:

var map = new BidirectionalDictionary<int, string>();
map.Add(1, "Alice");
map.Add(2, "Bob");

// Change key 1 to pair with a NEW value ("Charlie") — no conflict, sets 1 → "Charlie"
if(!map.TrySet(1, "Charlie", out int conflictKey))
    Console.WriteLine($"Conflict: 'Charlie' already owned by key {conflictKey}");
// map now: 1→"Charlie", 2→"Bob"

// "Bob" is already owned by key 2 — conflict
if(!map.TrySet(3, "Bob", out conflictKey))
    Console.WriteLine($"Conflict: 'Bob' already owned by key {conflictKey}"); // → key 2
// dictionary unchanged: still 1→"Charlie", 2→"Bob"

Return value semantics:

• true — the mapping is now set (either it was just written, or it was already exactly key → value). differentKeyOwns is always default.
• false — value is already owned by a different key. The dictionary is unchanged — no partial modifications, no evictions. differentKeyOwns is the key that owns value.

This is the right choice when:

• You want Force = false semantics (no silent evictions) but also don't want an exception — you need to handle the conflict gracefully in normal control flow
• The conflict is a recoverable, expected condition (e.g. user-driven data entry, protocol negotiation)
• You want to avoid the extra lookup you'd need to manually pre-check for a conflict before calling Set

AllowDefaults is still respected: null or default keys/values throw ArgumentNullException / ArgumentException regardless.

Retrieving Elements

// Get value by key (throws KeyNotFoundException if not found)
TValue value = map[key];
TValue value = map.GetValue(key);

// Get key by value (throws KeyNotFoundException if not found)
TKey key = map[value];         // reverse indexer
TKey key = map.GetKey(value);  // explicit method

// TryGet* variants - safe versions that don't throw
bool found = map.TryGetValue(key, out TValue value);
bool found = map.TryGetKey(value, out TKey key);

Removing Elements

// Remove by key
bool removed = map.RemoveByKey(key);
bool removed = map.Remove(key);  // alias for RemoveByKey

// Remove by value
bool removed = map.RemoveByValue(value);

// Clear all
map.Clear();

Checking Existence

bool hasKey = map.ContainsKey(key);
bool hasValue = map.ContainsValue(value);

Custom Comparers

// Case-insensitive lookups in both directions
var map = new BidirectionalDictionary<string, string>(
    keyComparer: StringComparer.OrdinalIgnoreCase,
    valueComparer: StringComparer.OrdinalIgnoreCase);

map.Add("US", "United States");
string name = map["us"];             // "United States"
string code = map["united states"];  // "US"

Collection Properties

int count = map.Count;
ICollection<TKey> keys = map.Keys;
ICollection<TValue> values = map.Values;

Enumeration

foreach (var pair in map)
{
    Console.WriteLine($"{pair.Key} -> {pair.Value}");
}

Use Cases

User ID to Username Mapping

BidirectionalDictionary<int, string> userMap = [];
userMap.Add(1001, "john_doe");
userMap.Add(1002, "jane_smith");

// Find user by ID
string username = userMap[1001];  // "john_doe"

// Find ID by username
int userId = userMap["jane_smith"];  // 1002

Country Code to Country Name

BidirectionalDictionary<string, string> countryMap = [];
countryMap.Add("US", "United States");
countryMap.Add("UK", "United Kingdom");
countryMap.Add("CA", "Canada");

// Get full name from code
string fullName = countryMap["US"];  // "United States"

// Get code from full name
string code = countryMap["Canada"];  // "CA"

Coordinate to Entity Mapping (Game Development)

BidirectionalDictionary<Vector3, GameObject> positionMap = [];
positionMap.Add(new Vector3(0, 0, 0), player);
positionMap.Add(new Vector3(10, 0, 5), enemy);

// Find entity at position
GameObject entity = positionMap[new Vector3(0, 0, 0)];

// Find position of entity
Vector3 position = positionMap[player];

Performance Characteristics

Space Complexity: The map maintains two internal dictionaries, so memory usage is approximately 2x that of a standard Dictionary.

Thread Safety

This collection is not thread-safe. If you need to access the map from multiple threads, you must provide your own synchronization mechanism (e.g., lock statements or ReaderWriterLockSlim).

Type Constraints

Both TKey and TValue must be non-nullable reference types or value types. The notnull constraint ensures type safety and prevents null-related runtime errors.

// Valid
BidirectionalDictionary<int, string> map1 = [];
BidirectionalDictionary<Guid, MyClass> map2 = [];

// Invalid - nullable types not allowed
// BidirectionalDictionary<int?, string> map3 = [];

Note that while 0, Guid.Empty, and similar are valid by default, you can opt in to rejecting them via AllowDefaults = false — see the Default Value Guard section.

Requirements

• .NET 8.0 or later (also targets .NET 10.0)
• C# 12 or later

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

1. Fork the repository
2. Create your feature branch (git checkout -b feature/amazing-feature)
3. Commit your changes (git commit -m 'Add some amazing feature')
4. Push to the branch (git push origin feature/amazing-feature)
5. Open a Pull Request

License

This project is licensed under the MIT License - see the LICENSE file for details.

Support

If you encounter any issues or have questions, please file an issue on the GitHub repository.