Kodster.Results 0.1.0

Kodster.Results

A result type for expected failures, so they travel as return values instead of exceptions.

No dependencies. Safe to reference from a domain or core project without dragging in a DI container, a validation library, or a web framework.

dotnet add package Kodster.Results

Targets .NET 10.

Result and Result<T>

Result represents an operation that either succeeded or failed with an Error. Result<T> adds a value on the success side.

Result<User> Find(Guid id)
{
    var user = _users.SingleOrDefault(u => u.Id == id);

    return user is null
        ? Error.NotFound("User.NotFound", "User was not found.")
        : user;
}

Neither branch needs a wrapper call. Error converts implicitly to Result and Result<T>, and any TValue converts implicitly to a successful Result<TValue>.

Reading one back:

var result = Find(id);

if (result.IsFailure)
{
    Console.WriteLine(result.Error.Description);
    return;
}

Use(result.Value);

Or branch in a single expression with Match:

var message = result.Match(
    onSuccess: user => $"Found {user.Nickname}",
    onFailure: error => $"Failed: {error.Code}");

Errors

An Error is a record of a code, a human-readable description, and an ErrorType that says what kind of failure it is.

Error.Conflict("OAuth.AlreadyLinked", "That provider is already linked to this account.")

Each has a matching factory taking a code and description. The set covers outcomes application code decides; it deliberately excludes failures that belong to the transport layer.

ErrorType is serialised by ordinal value, so new members are only ever appended. Inserting one would change the number every later type serialises as.

Error.None is the sentinel carried by successful results. Constructing a success with an error, or a failure without one, throws.

Validation results

ValidationResult and ValidationResult<T> are failures that carry many errors rather than one — the natural shape for input validation, where you want every broken rule at once.

Error[] errors = [
    Error.Validation("Nickname", "Nickname is required."),
    Error.Validation("Age", "Age must be 13 or older.")
];

return ValidationResult<UserResponse>.WithErrors(errors);

Both implement IValidationResult, so a caller can pattern-match to decide whether to report one error or all of them:

Error[] toReport = result switch
{
    IValidationResult validation => validation.Errors,
    _ => [result.Error]
};

They are otherwise ordinary failed results, with Error set to a general Validation.Error.

Companion packages

• Kodster.CQRS — request/handler abstraction returning these result types, with validation and logging decorators.
• Kodster.Results.AspNetCore — maps a failed Result onto an HTTP ProblemDetails response.

License

MIT