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

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