Functional Extensions for C#

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

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(", ", fruits)) // "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"

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();

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

Read or Watch more about these ideas

• Functional C#: Primitive obsession
• Functional C#: Non-nullable reference types
• Functional C#: Handling failures, input errors
• Applying Functional Principles in C# Pluralsight course

Related Projects

• Typescript Functional Extensions
• UniTask extensions for Unity

Contributors

A big thanks to the project contributors!

• Alexey Malinin
• Robert Larkins
• tinytownsoftware
• piotr121993
• Dmitry Korotin
• michalsznajder
• Xavier
• Julien Aspirot
• Kyle McMaster
• Vinícius Beloni Cubas
• rutkowskit
• Giovanni Costagliola
• Mark Wainwright
• ProphetLamb
• Paul Williams
• alexmurari
• ruud
• Tomasz Malinowski
• Staffan Wingren
• Tim Schneider
• Piotr Karasiński
• Marcel Roozekrans
• guythetechie
• Logan Kahler
• Ali Khalili
• Andrei Andreev
• YudApps
• dataphysix
• Laszlo Lueck
• Sean G. Wright
• Samuel Viesselman
• Stian Kroknes
• dataneo
• michaeldileo
• Renato Ramos Nascimento
• Patrick Drechsler
• Vadim Mingazhev
• Darick Carpenter
• Stéphane Mitermite
• Markus Nißl
• Adrian Frielinghaus
• svroonland
• JvSSD
• mnissl
• Vladimir Makaev
• Ben Smith
• pedromtcosta
• Michał Bator
• mukmyash
• azm102
• ThomasDC
• bopazyn
• Joris Goovaerts
• Ivan Deev
• Damian Płaza
• ergwun
• Michael DiLeo
• Jean-Claude
• Matt Jenkins
• Michael Altmann
• Steven Giesel
• Anton Hryshchanka
• Mikhail Bashurov
• kostekk88
• Carl Abrahams
• golavr
• Sviataslau Hankovich
• Chad Gilbert
• Robert Sęk
• Sergey Solomentsev
• Malcolm J Harwood
• Dragan Stepanovic
• Ivan Novikov
• Denis Molokanov
• Gerald Wiltse
• yakimovim
• Alex Erygin
• Omar Aloraini