Option

The Option type is a powerful construct used to represent the presence or absence of a value. This type is particularly useful for eliminating null reference errors, a common source of bugs in C# applications.

An Option can be either Some (containing a value), or None (indicating the absence of a value). By using Option, developers can enforce safer code practices, making it explicit when a variable, property, or parameter might not have a value and requires handling both cases.

For error handling, you can use the Result type, which can encapsulate the error information.

Overview

The Option type is a container for a value, so it can be treated as a collection with a single value supporting operations such as:

• Mapping: apply transformations to the value within a Option
• Filtering: filters a value based on a predicate
• Flattening: apply a function that returns an Option and flattens the result

Usage

The following example demonstrates how to use Option to handle non-whitespace strings:

public Option<string> OnlyNonWhitespace(string? value) =>
    string.IsNullOrWhiteSpace(value)
        ? None
        : Some(value);

In this example:

• The function OnlyNonWhitespace takes an input string that may be null or contain whitespace.
• If the input string is null, empty or contains only whitespace, the function returns None, indicating the absence of a valid string.
• If the input string is non-whitespace, the function returns Some with the actual string value.

Accessors and Unwrapping

Functions that are used to access or extract values from their containers.

TryGet / Unwrap

You can use TryGet as an escape hatch to get the value out.

Option<string> nonWhitespace = OnlyNonWhitespace("hello");

if (nonWhitespace.TryGet(out var value))
{
    Console.WriteLine(value);
}

// Alternatively, `Unwrap` can be used,
// but will throw an exception if in the `None` state.
string value = nonWhitespace.Unwrap();

Or you can use UnwrapOr / UnwrapOrElse and specify a fallback value for when option is empty.

Option<string> nonWhitespace = OnlyNonWhitespace("hello");

string value = nonWhitespace.UnwrapOr("<undefined>");

ToNullable

The ToNullable / ToNullableValue is used to unwrap the value from option and fallback to null when it is empty.

Option<string> nonWhitespace = OnlyNonWhitespace("hello");

string? value = nonWhitespace.ToNullable();

Pattern Matching and Transformation

Functions that are used to apply transformations or perform pattern matching on values within containers.

Match

To handle the possible states of the option:

int length = OnlyNonWhitespace("  hello  ")
    .Map(x => x.Trim())
    .Match(
        Some: x => x.Length,
        None: () => 0);

There are shortcuts for a lot of these - for example, the above could be written as:

int length = OnlyNonWhitespace("  hello  ")
    .Map(x => x.Trim())
    .Map(x => x.Length)
    .UnwrapOr(0);

OkOr / OkOrElse

You can turn Options into other types, such as Result:

Result<int, string> result = OnlyNonWhitespace("  hello  ")
    .Map(x => x.Trim())
    .Where(x => x.Length > 0)
    .OkOr("That string was only whitespace! Bad!")

ValidOr / ValidOrElse

To convert an Option to Validation:

Validation<int, string> result = Some("Hello")
    .Where(x => x.Length < 10)
    .ValidOr("Too long text");

Filtering and Conditional Operators

Functions that are used to filter or conditionally manipulate values within containers.

Where

Filter the element of an option, if any, based on a predicate.

Option<int> nonZeroLength = OnlyNonWhitespace("  hello  ")
    .Map(x => x.Trim())
    .Map(x => x.Length)
    .Where(x => x.Length > 0);

OfType

Use OfType to filter the value held in Some based on its type.

This is essentially a shorthand for FlatMap(a => a is U u ? Some(u) : None)

Mapping and Flat Mapping

Functions that are used to apply transformations to each element within a container and manage nested containers.

Map / Select

Transforming the value:

OnlyNonWhitespace("  hello  ")
    .Map(x => x.Trim());

LINQ syntax is supported too.

Option<int> result =
    from a in Some(3)
    select a + 1;

FlatMap / SelectMany

Monadic bind (also called flat mapping):

Option<string> greeting = OnlyNonWhitespace("  hello  ")
    .Map(x => x.Trim())
    .FlatMap(a =>
        OnlyNonWhitespace("   world   ")
            .Map(y => y.Trim())
            .Map(b => $"{a} {b}");

That wasn't very pretty - you can use LINQ to make it nicer:

Option<string> greeting =
    from a in OnlyNonWhitespace("  hello  ").Map(x => x.Trim())
    from b in OnlyNonWhitespace("   world   ").Map(y => y.Trim())
    select $"{a} {b}";

Do

Use Do to execute an imperative operation when the option has a value.

Some("Hello")
    .Do(x => Console.WriteLine(x));

Aggregation and Collection Operations

Functions that are used to aggregate or collect values from multiple containers.

Somes

Use the Somes to extract the values from a sequence of options.

IEnumerable<int> numbers = ListOf
    .Many(Some(4), Some(3), None, Some(10), None, None, Some(2))
    .AsEnumerable()
    .Somes()

SomesMap

You can use SomesMap as a shortcut for Somes + Map.

IEnumerable<int> lengths = ListOf
    .Many(Some("Hello"), Some("world"))
    .SomesMap(v => v.Length)

Traverse

You can traverse between various other container types. For example:

IReadOnlyList<int> list = [2, 4, 6];

Option<IReadOnlyList<int>> listOfOnlyEvenNumbers =
    list.Traverse(x => x % 2 == 0 ? Some(x) : None);
// Some([2, 4, 6])

Sequence

Use Sequence to traverse without the mapping step.

This is equivalent to Traverse(Identity) / Traverse(x => x)

IReadOnlyList<Option<int>> list = [Some(2), Some(4), None, Some(6)];

Option<IReadOnlyList<int>> sequenced =
    list.Sequence();

Prelude

The Prelude class provides the following functions for Option:

Some / None

Returns a wrapped value or an empty Option.

public Option<string> OnlyNonWhitespace(string? value) =>
    string.IsNullOrWhiteSpace(value)
        ? None
        : Some(value);

Optional

Use Optional to convert a nullable value to Option. If the input value is not null, the result will be Some(value), otherwise the result will be an empty option (None).

string? value = "Hello";
Option<string> wrapped = Optional(value);