Optional2

Optional is a robust option/maybe type for C#. Originally developed by Nils Lück - Optional

What and Why?

Optional is a strongly typed alternative to null values that lets you:

• Avoid those pesky null-reference exceptions
• Signal intent and model your data more explictly
• Cut down on manual null checks and focus on your domain
• Work with optional values of reference and value types in the same way. Two versions of the same generic method with where T : class and where T : struct are no longer needed

Installation

PM> Install-Package Optional2

NuGet package. Supports .NET 3.5+ and .NET (.NET Standard 1.0+)

Usage

Using the library

To use Optional simply import the following namespace:

using Optional;

A few auxiliary namespaces are provided:

using Optional.Linq; // Linq query syntax support
using Optional.Unsafe; // Unsafe value retrieval
using Optional.Collections; // Linq like methods with Option specifics

Creating optional values

// The most basic way to create optional values is to use the static `Option` class:
var none = Option.None<int>();
var some = Option.Some(10);
// or use extension methods:
var none = 10.None(); // Equivalent to Option.None<int>()
var some = 10.Some();

Option can be filtered during creation by methods .Some*() or .None*(). The most useful from them is .SomeNotNull() since Nullable Reference Types are supported. Analogue for value types is .ToOption().

Retrieving values

When retrieving values, Optional forces you to consider both cases (that is if a value is present or not).

Like Nullable<T> Option can be tested by HasValue property. There are also more precise ways:

var isThousand = option.Contains(1000);
var isGreaterThanThousand = option.Exists(val => val > 1000);

Ways to retrieve a value from Option are:

var value = option.ValueOr(10); // Returns the value if Some, or otherwise an alternative value (10)
var value = option.Match(
  some: x => x + 1, 
  none: () => 10
); // pattern matching
var value = option.ValueOrFailure(); // Unsafe: throws OptionValueMissingException on None

Transforming and filtering values

var value = 10.Some();
var doubled = value.Map(x => x * 2); // Some(20)
var odd = doubled.Filter(x => x % 2 == 1); // None
var fallback = odd.Else(1.Some()); // Some(1)

For details see Option<T> or explore xml doc.

Other utilities

Monadic evaluation with LINQ query syntax

using Optional.Linq;

var personWithGreenHair =
  from person in FindPersonById(10)
  from hairstyle in GetHairstyle(person)
  where hairstyle.Color == "green"
  select person;

Options with exceptional values aka Either

An Option<T, TException> type with similar capabilities.

var none = Option.None<int, ErrorCode>(ErrorCode.GeneralError);
var some = Option.Some<int, ErrorCode>(10);

Working with collections

• IEnumerable<T> related Linq similar methods (First|Last|Signle)OrNone()
• items.Values() keeps only Some from items and unwraps them
• dictionary.GetValueOrNone(key: 42) - lookups an entry by key