Wrap.Either.Testing 2.0.0-beta.3

Wrap: Functional Monads for .NET

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Functional monads for C#, to handle null values and failures without exceptions. Chain operations with method calls or LINQ query syntax (railway-oriented programming), with async support throughout.

Types

• Maybe<T>: either Some<T> or None. Use it instead of null.
• Result<T>: either Ok<T> or Failure. Use it instead of throwing exceptions.
• Either<TLeft, TRight>: either Left<TLeft> or Right<TRight>.
• Id<TId, TValue>: strongly typed IDs backed by int, uint, long, ulong or Guid.

Maybe<T>, Result<T> and Either<TLeft, TRight> are C# 15 union types. Each is a struct, so:

• switch and is patterns work on the cases, and the compiler checks that a switch covers them all.
• Wrapping a value in Some<T> or Ok<T> does not allocate.
• They are never null. default(Maybe<T>) is None, and default(Result<T>) is a Failure.

Example

using Wrap;
using Wrap.Extensions;

static Result<int> ParseAge(string input) =>
    int.TryParse(input, out var age) && age >= 0
        ? age
        : R.Fail("Age must be a whole number of 0 or more.");

static string Describe(Result<int> age) =>
    age.Map(x => x + 1) switch
    {
        Ok<int>(var next) => $"Next year you'll be {next}.",
        Failure f => f.Message
    };

static string Greet(string? name) =>
    M.Wrap(name) // null becomes None
        .Map(x => $"Hello, {x}!")
        .Match(fNone: () => "Hello, stranger!", fSome: x => x);

View the Wiki for full documentation.

Packages

The core types are also published on their own: Wrap.Common, Wrap.Maybe, Wrap.Result, Wrap.Either and Wrap.Id, each with a matching .Testing package.

Requirements

Wrap 2 targets .NET 11 and needs C# 15, which comes with the .NET 11 SDK.

Wrap 1.x supports .NET 8, 9, 10 and 11, so you can stay on it without union types. It is in maintenance and gets fixes only.

History

Wrap brings together projects I have been working on since 2019. It started with a Result<T> implementation, which became Maybe (v10 in November 2025), and I decided it was best to have both.

At the same time I was maintaining StrongId (v8.5 in November 2024) so entity IDs didn't have to be primitives. I realised how much Result<T>, Maybe<T> and Id<T> had in common and brought them together in one library: Wrap.

Version 2 rebuilds the core types on the union types introduced in C# 15.

Licence

MIT

Copyright

Copyright (c) 2019-2026 bfren (unless otherwise stated)