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Functional Extensions for C#

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This library helps write code in more functional way. To get to know more about the principles behind it, check out the Applying Functional Principles in C# Pluralsight course.

Installation

Available on NuGet

dotnet add package CSharpFunctionalExtensions

or

PM> Install-Package CSharpFunctionalExtensions

Also available as a strong named assembly (big thanks to bothzoli who made it possible!).

On NuGet

dotnet add package CSharpFunctionalExtensions.StrongName

Core Concepts

Get rid of primitive obsession

Result<CustomerName> name = CustomerName.Create(model.Name); Result<Email> email = Email.Create(model.PrimaryEmail); Result result = Result.Combine(name, email); if (result.IsFailure) return Error(result.Error); var customer = new Customer(name.Value, email.Value);

Make nulls explicit with the Maybe type

Maybe<Customer> customerOrNothing = _customerRepository.GetById(id); if (customerOrNothing.HasNoValue) return Error("Customer with such Id is not found: " + id);

Compose multiple operations in a single chain

return _customerRepository.GetById(id) .ToResult("Customer with such Id is not found: " + id) .Ensure(customer => customer.CanBePromoted(), "The customer has the highest status possible") .Tap(customer => customer.Promote()) .Tap(customer => _emailGateway.SendPromotionNotification(customer.PrimaryEmail, customer.Status)) .Finally(result => result.IsSuccess ? Ok() : Error(result.Error));

Wrap multiple operations in a TransactionScope

return _customerRepository.GetById(id) .ToResult("Customer with such Id is not found: " + id) .Ensure(customer => customer.CanBePromoted(), "The customer has the highest status possible") .WithTransactionScope(customer => Result.Success(customer) .Tap(customer => customer.Promote()) .Tap(customer => customer.ClearAppointments())) .Tap(customer => _emailGateway.SendPromotionNotification(customer.PrimaryEmail, customer.Status)) .Finally(result => result.IsSuccess ? Ok() : Error(result.Error));

API Examples

Maybe

Explicit Construction

Use case: Creating a new Maybe containing a value

Maybe<string> apple = Maybe<string>.From("apple"); // or Maybe<string> apple = Maybe.From("apple"); // type inference // or var apple = Maybe.From("apple");

None/No Value

Use case: Replacing null or the Null Object Pattern for representing 'missing' data.

int storeInventory = ... Maybe<string> fruit = storeInventory > 0 ? Maybe<string>.From("apple") : Maybe<string>.None; // or where the generic type is a reference type Maybe<string> fruit = null; // or where the generic type is a value type Maybe<int> fruit = default;

Implicit Conversion

Use case: Easily creating a Maybe from a value

// Constructing a Maybe Maybe<string> apple = "apple"; // implicit conversion // Or as a method return value Maybe<string> GetFruit(string fruit) { if (string.IsNullOrWhiteSpace(fruit)) { return Maybe<string>.None; } return fruit; // implicit conversion }

Equality

Use case: Comparing Maybes or values without knowledge of the inner value of the Maybes

Maybe<string> apple = "apple"; Maybe<string> orange = "orange"; string alsoOrange = "orange"; Maybe<string> noFruit = Maybe<string>.None; Console.WriteLine(apple == orange); // false Console.WriteLine(apple != orange); // true Console.WriteLine(orange == alsoOrange); // true Console.WriteLine(alsoOrange == noFruit); // false

ToString

Maybe<string> apple = "apple"; Maybe<string> noFruit = Maybe<string>.None; Console.WriteLine(apple.ToString()); // "apple" Console.WriteLine(noFruit.ToString()); // "No value"

GetValueOrThrow

Use case: Procedurally accessing the inner value of the Maybe

Note: Calling this will throw a InvalidOperationException if there is no value

Maybe<string> apple = "apple"; Maybe<string> noFruit = Maybe<string>.None; Console.WriteLine(apple.GetValueOrThrow()); // "apple"; Console.WriteLine(noFruit.GetValueOrThrow()); // throws InvalidOperationException !! Console.WriteLine(noFruit.GetValueOrThrow(new CustomException())); // throws CustomException !!

HasValue and HasNoValue

Use case: Procedurally checking if the Maybe has a value, usually before accessing the value directly

void Response(string fruit) { Console.WriteLine($"Yum, a {fruit} 😀"); } Maybe<string> apple = "apple"; Maybe<string> noFruit = Maybe<string>.None; if (apple.HasValue) { Response(apple.Value); // safe to access since we checked above } if (noFruit.HasNoValue) { Response("We're all out of fruit 😢"); }

GetValueOrDefault

Use case: Safely accessing the inner value, without checking if there is one, by providing a fallback if no value exists

void Response(string fruit) { Console.WriteLine($"It's a {fruit}"); } Maybe<string> apple = "apple"; Maybe<string> unknownFruit = Maybe<string>.None; string appleValue = apple.GetValueOrDefault("banana"); string unknownFruitValue = unknownFruit.GetValueOrDefault("banana"); Response(appleValue); // It's a apple Response(unknownFruitValue); // It's a banana

Where

Use case: Converting a Maybe with a value to a Maybe.None if a condition isn't met

Note: The predicate passed to Where (ex )

bool IsMyFavorite(string fruit) { return fruit == "papaya"; } Maybe<string> apple = "apple"; Maybe<string> favoriteFruit = apple.Where(IsMyFavorite); Console.WriteLine(favoriteFruit.ToString()); // "No value"

Map

Use case: Transforming the value in the Maybe, if there is one, without needing to check if the value is there

Note: the delegate (ex CreateMessage) passed to Maybe.Map() is only executed if the Maybe has an inner value

string CreateMessage(string fruit) { return $"The fruit is a {fruit}"; } Maybe<string> apple = "apple"; Maybe<string> noFruit = Maybe<string>.None; Console.WriteLine(apple.Map(CreateMessage).Unwrap("No fruit")); // "The fruit is a apple" Console.WriteLine(noFruit.Map(CreateMessage).Unwrap("No fruit")); // "No fruit"

Select

Alias: Maybe.Select() is an alias of Maybe.Map()

Bind

Use case: Transforming from one Maybe into another Maybe (like Maybe.Map but it transforms the Maybe instead of the inner value)

Note: the delegate (ex MakeAppleSauce) passed to Maybe.Bind() is only executed if the Maybe has an inner value

Maybe<string> MakeAppleSauce(Maybe<string> fruit) { if (fruit == "apple") // we can only make applesauce from apples 🍎 { return "applesauce"; } return Maybe<string>.None; } Maybe<string> apple = "apple"; Maybe<string> banana = "banana"; Maybe<string> noFruit = Maybe<string>.None; Console.WriteLine(apple.Bind(MakeAppleSauce).ToString()); // "applesauce" Console.WriteLine(banana.Bind(MakeAppleSauce).ToString()); // "No value" Console.WriteLine(noFruit.Bind(MakeAppleSauce).ToString()); // "No value"

SelectMany

Alias: Maybe.SelectMany() is an alias of Maybe.Bind()

Choose

Use case: Filter a collection of Maybes to only the ones that have a value, and then return the value for each, or map that value to a new one

Note: the delegate passed to Maybe.Choose() is only executed on the Maybes of the collection with an inner value

IEnumerable<Maybe<string>> unknownFruits = new[] { "apple", Maybe<string>.None, "banana" }; IEnumerable<string> knownFruits = unknownFruits.Choose(); IEnumerable<string> fruitResponses = unknownFruits.Choose(fruit => $"Delicious {fruit}"); Console.WriteLine(string.Join(", ", knownFruits)) // "apple, banana" Console.WriteLine(string.Join(", ", fruitResponses)) // "Delicious apple, Delicious banana"

Execute

Use case: Safely executing a void (or Task) returning operation on the Maybe inner value without checking if there is one

Note: the Action (ex PrintFruit) passed to Maybe.Execute() is only executed if the Maybe has an inner value

void PrintFruit(string fruit) { Console.WriteLine($"This is a {fruit}"); } Maybe<string> apple = "apple"; Maybe<string> noFruit = Maybe<string>.None; apple.Execute(PrintFruit); // "This is a apple" noFruit.Execute(PrintFruit); // no output to the console

ExecuteNoValue

Use case: Executing a void (or Task) returning operation when the Maybe has no value

void LogNoFruit(string fruit) { Console.WriteLine($"There are no {fruit}"); } Maybe<string> apple = "apple"; Maybe<string> banana = Maybe<string>.None; apple.ExecuteNoValue(() => LogNoFruit("apple")); // no output to console banana.ExecuteNoValue(() => LogNoFruit("banana")); // "There are no banana"

Or

Use case: Supplying a fallback value Maybe or value in the case that the Maybe has no inner value

Note: The fallback Func<T> (ex () => "banana") will only be executed if the Maybe has no inner value

Maybe<string> apple = "apple"; Maybe<string> banana = "banana"; Maybe<string> noFruit = Maybe<string>.None; Console.WriteLine(apple.Or(banana).ToString()); // "apple" Console.WriteLine(noFruit.Or(() => banana)).ToString()); // "banana" Console.WriteLine(noFruit.Or("banana").ToString()); // "banana" Console.WriteLine(noFruit.Or(() => "banana").ToString()); // "banana"

Match

Use case: Defining two operations to perform on a Maybe. One to be executed if there is an inner value, and the other to executed if there is not

Maybe<string> apple = "apple"; Maybe<string> noFruit = Maybe<string>.None; // Void returning Match apple.Match( fruit => Console.WriteLine($"It's a {fruit}"), () => Console.WriteLine("There's no fruit")); // Mapping Match string fruitMessage = noFruit.Match( fruit => $"It's a {fruit}", () => "There's no fruit")); Console.WriteLine(fruitMessage); // "There's no fruit"

TryFirst and TryLast

Use case: Replacing .FirstOrDefault() and .LastOrDefault() so that you can return a Maybe instead of a null or value type default value (like 0, false) when working with collections

IEnumerable<string> fruits = new[] { "apple", "coconut", "banana" }; Maybe<string> firstFruit = fruits.TryFirst(); Maybe<string> probablyABanana = fruits.TryFirst(fruit => fruit.StartsWith("ba")); Maybe<string> aPeachOrAPear = fruits.TryFirst(fruit => fruit.StartsWith("p")); Console.WriteLine(firstFruit.ToString()); // "apple" Console.WriteLine(probablyABanana.ToString()); // "banana" Console.WriteLine(aPeachOrAPear.ToString()); // "No value" Maybe<string> lastFruit = fruits.TryLast(); Maybe<string> anAppleOrApricot = fruits.TryLast(fruit => fruit.StartsWith("a")); Console.WriteLine(lastFruit.ToString()); // "banana" Console.WriteLine(anAppleOrApricot.ToString()); // "apple"

TryFind

Use case: Safely getting a value out of a Dictionary

Dictionary<string, int> fruitInventory = new() { { "apple", 10 }, { "banana", 2 } }; Maybe<int> appleCount = fruitInventory.TryFind("apple"); Maybe<int> kiwiCount = fruitInventory.TryFind("kiwi"); Console.WriteLine(appleCount.ToString()); // "10" Console.WriteLine(kiwiCount.ToString()); // "No value"

ToResult

Use case: Representing the lack of an inner value in a Maybe as a failed operation

Note: See Result section below

Maybe<string> fruit = "banana"; Maybe<string> noFruit = Maybe<string>.None; string errorMessage = "There was no fruit to give"; Result<string> weGotAFruit = fruit.ToResult(errorMessage); Result<string> failedToGetAFruit = noFruit.ToResult(errorMessage); Console.WriteLine(weGotAFruit.Value); // "banana" Console.WriteLine(failedToGetAFruit.Error); // "There was no fruit to give"

ToUnitResult

Use case: Representing the lack of an inner value in a Maybe as a failed operation, if an Error is provided

Use case: Representing the presence of an inner value in a Maybe as a failed operation

Note: See UnitResult section below

Maybe<Error> error = new Error(); Maybe<string> noFruit = Maybe<string>.None; UnitResult<Error> weGotAnError = error.ToUnitResult(); UnitResult<Error> failedToGetAFruit = noFruit.ToUnitResult(new Error()); Console.WriteLine(weGotAnError.IsFailure); // true Console.WriteLine(failedToGetAFruit.IsFailure); // true

Result

Explicit Construction: Success and Failure

Use case: Creating a new Result in a Success or Failure state

record FruitInventory(string Name, int Count); Result<FruitInventory> appleInventory = Result.Success(new FruitInventory("apple", 4)); Result<FruitInventory> failedOperation = Result.Failure<FruitInventory>("Could not find inventory"); Result successInventoryUpdate = Result.Success();

To create a success result of a value you can also use the Of method which has overloads for Func<T> and Task<T>.

Result<Something> something = Result.Of(_service.CreateSomething()); Result<Something> something = await Result.Of(_service.CreateSomethingAsync()); Result<Something> something = Result.Of(() => _service.CreateSomething()); Result<Something> something = await Result.Of(() => _service.CreateSomethingAsync());

Conditional Construction: SuccessIf and FailureIf

Use case: Creating successful or failed Results based on expressions or delegates instead of if/else statements or ternary expressions

bool onTropicalIsland = true; Result foundCoconut = Result.SuccessIf(onTropicalIsland, "These trees seem bare 🥥"); Result foundGrapes = Result.FailureIf(() => onTropicalIsland, "No grapes 🍇 here"); // or bool isNewShipmentDay = true; Result<FruitInventory> appleInventory = Result.SuccessIf(isNewShipmentDay, new FruitInventory("apple", 4), "No 🍎 today"); Result<FruitInventory> bananaInventory = Result.SuccessIf(() => isNewShipmentDay, new FruitInventory("banana", 2), "All out of 🍌"); // or bool afterBreakfast = true; Result<FruitInventory> orangeInventory = Result.FailureIf(afterBreakfast, new FruitInventory("orange", 10), "No 🍊 today"); Result<FruitInventory> grapefruitInventory = Result.FailureIf(() => afterBreakfast, new FruitInventory("grapefruit", 5), "No grapefruit 😢");

Implicit Conversion

Use case: Easily creating a successful result from a value

Result<FruitInventory> appleInventory = new FruitInventory("apple", 4); Result failedInventoryUpdate = "Could not update inventory";

ToString

Use case: Printing out the state of a Result and its inner value or error

Result<FruitInventory> appleInventory = new FruitInventory("apple", 4); Result<FruitInventory> bananaInventory = Result.Failure<FruitInventory>("Could not find any bananas"); Result failedInventoryUpdate = "Could not update inventory"; Result successfulInventoryUpdate = Result.Success(); Console.WriteLine(appleInventory.ToString()); // "Success(FruitInventory { Name = apple, Count = 4 })" Console.WriteLine(bananaInventory.ToString()); // "Failure(Could not find any bananas)" Console.WriteLine(failedInventoryUpdate.ToString()); // "Failure(Could not update inventory)" Console.WriteLine(successfulInventoryUpdate.ToString()); // "Success"

Map

Use case: Transforming the inner value of a successful Result, without needing to check on the success/failure state of the Result

Note: the delegate (ex CreateMessage) passed to Result.Map() is only executed if the Result was successful

string CreateMessage(FruitInventory inventory) { return $"There are {inventory.Count} {inventory.Name}(s)"; } Result<FruitInventory> appleInventory = new FruitInventory("apple", 4); Result<FruitInventory> bananaInventory = Result.Failure<FruitInventory>("Could not find any bananas"); Console.WriteLine(appleInventory.Map(CreateMessage).ToString()); // "Success(There are 4 apple(s))" Console.WriteLine(bananaInventory.Map(CreateMessage).ToString()); // "Failure(Could not find any bananas)"

MapError

Use case: Transforming the inner error of a failed Result, without needing to check on the success/failure state of the Result

Note: the delegate (ex ErrorEnhancer) passed to Result.MapError() is only executed if the Result failed

string ErrorEnhancer(string errorMessage) { return $"Failed operation: {errorMessage}"; } Console.WriteLine(appleInventory.MapError(ErrorEnhancer).ToString()); // "Success(FruitInventory { Name = apple, Count = 4 })" Console.WriteLine(bananaInventory.MapError(ErrorEnhancer).ToString()); // "Failed operation: Could not find any bananas"

Testing

CSharpFunctionalExtensions.FluentAssertions

A small set of extensions to make test assertions more fluent when using CSharpFunctionalExtensions! Check out the repo for this library more information!

Includes custom assertions for

  • Maybe
  • Result
  • Result
  • Result<T, E>
  • UnitResult

Example

var result = Result.Success(420); result.Should().Succeed(); // passes result.Should().SucceedWith(420); // passes result.Should().SucceedWith(69); // throws result.Should().Fail(); // throws

Web APIs / HttpResults

CSharpFunctionalExtensions.HttpResults

This library provides convenient extension methods to seamlessly map Results from CSharpFunctionalExtensions to HttpResults. With this, it streamlines your Web API resulting in cleaner, more fluent code.

Key Benefits

  • ⚙️ Zero Configuration: Get started immediately — the mapping works out of the box without any configuration.
  • 🛠️ Customizable Mappings: Tailor default mappings or define custom mappings for specific use cases.
  • 🔗 Fluent API: Maintain a smooth, railway-oriented flow by chaining HttpResult mappings at the end of your Result chain.
  • 🧱 Separation of Domain and HTTP Errors: Keeps domain errors distinct from HTTP errors, improving maintainability and clarity between business logic and web API concerns.
  • ⚡ Minimal APIs & Controllers Support: Works with both Minimal APIs and traditional controllers in ASP.NET.
  • 📦 Full Support for ASP.NET Results: Supports all built-in HTTP response types in ASP.NET, including Ok, Created, NoContent, Accepted, FileStream, and more.
  • 🦺 Typed Results: Utilizes TypedResults for consistent, type-safe API responses.
  • 📑 OpenAPI Ready: Ensures accurate OpenAPI generation for clear and reliable API documentation.
  • 🛡️ RFC Compliance: Default mappings adhere to the RFC 9457 standard (ProblemDetails), ensuring your API errors are standardized and interoperable.
  • 🧑‍💻 Developer-Friendly: Includes built-in analyzers and source generators to speed up development and reduce errors.

Example

app.MapGet("/books", (BookService service) => service.Get() //Result<Book[]> .ToOkHttpResult() //Results<Ok<Book[]>, ProblemHttpResult> );

Check the repo out here.

Analyzers

CSharpFunctionalExtensions.Analyzers

A Roslyn analyzer package that provides warnings and recommendations to prevent misuse of Result objects in CSharpFunctionalExtensions. Ensures more robust implementation when working with Result types.

Available on NuGet

dotnet add package CSharpFunctionalExtensions.Analyzers

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Contributors

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Functional extensions for C#

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csharp functional-programming entity result value-object maybe-monad

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