LightResults is an extremely light and modern .NET library that provides a simple and flexible
implementation of the Result Pattern. The Result Pattern is a way of representing the outcome
of an operation, whether it's successful or has encountered an error, in a more explicit and
structured manner. This project is heavily inspired by Michael Altmann's
excellent work with FluentResults.
References
This library targets .NET Standard 2.0, .NET 6.0, .NET 7.0, .NET 8.0, and .NET 9.0.
Dependencies
This library has no dependencies.
Advantages of this library
🪶 Lightweight — Only contains what's necessary to implement the Result Pattern.
⚙️ Extensible — Simple interfaces and base classes make it easy to adapt.
🧱 Immutable — Results and errors are immutable and cannot be changed after being created.
🧵 Thread-safe — The Error list and Metadata dictionary use Immutable classes for thread-safety.
✨ Modern — Built against the latest version of .NET using the most recent best practices.
🧪 Native — Written, compiled, and tested against the latest versions of .NET.
❤️ Compatible — Available for dozens of versions of .NET as a
.NET Standard 2.0 library.
LightResults consists of only three classes Result, Result<TValue>, and Error.
The Result class represents a generic result indicating success or failure.
The Result<TValue> class represents a success or failure result with a value.
The Error class represents an error with a message and optional associated metadata.
Creating a successful result
Successful results can be created using the Success method.
var successResult = Result.Success();
var successResultWithValue = Result.Success(349.4);
Creating a failure result
Failed results can be created using the Failure method.
var failureResult = Result.Failure();
var failureResultWithMessage = Result.Failure("Operation failure!");
var failureResultWithMessageAndMetadata = Result.Failure("Operation failure!", ("UserId", userId));
var failureResultWithMessageAndException = Result.Failure("Operation failure!", ex);
Checking the state of a result
There are two methods used to check a result, IsSuccess() and IsFailed(). Both of which have several overloads to obtain the
value and error.
if (result.IsSuccess())
{
// The result is successful.
}
if (result.IsFailure(out var error))
{
// The result is failure.
if (error.Message.Length > 0)
Console.WriteLine(error.Message);
else
Console.WriteLine("An unknown error occured!");
}
Getting the value
The value from a successful result can be retrieved through the out parameter of the Success() method.
if (result.IsSuccess(out var value))
{
Console.WriteLine($"Value is {value}");
}
Creating errors
Errors can be created with or without a message.
var errorWithoutMessage = new Error();
var errorWithMessage = new Error("Something went wrong!");
Or with a message and metadata.
var errorWithMetadataTuple = new Error("Something went wrong!", ("Key", "Value"));
var metadata = new Dictionary<string, object> { { "Key", "Value" } };
var errorWithMetadataDictionary = new Error("Something went wrong!", metadata);
Custom errors
The best way to represent specific errors is to make custom error classes that inherit from Error
and define the error message as a base constructor parameter.
public sealed class NotFoundError : Error
{
public NotFoundError()
: base("The resource cannot be found.")
{
}
}
var notFoundError = new NotFoundError();
var notFoundResult = Result.Failure(notFoundError);
Then the result can be checked against that error type.
if (result.IsFailure(out var error) && error is NotFoundError)
{
// Looks like the resource was not found, we better do something about that!
}
Or checked to see if there are any errors of that type.
if (result.IsFailure() && result.HasError<NotFoundError>())
{
// At least one of the errors was a NotFoundError.
}
This can be especially useful when combined with metadata that is related to a specific type of error.
public sealed class HttpError : Error
{
public HttpError(HttpStatusCode statusCode)
: base("An HTTP error occured.", ("StatusCode", statusCode))
{
}
}
We can further simplify creating errors by creating an error factory.
public static AppError
{
public Result NotFound()
{
var notFoundError = new NotFoundError();
return Result.Failure(notFoundError);
}
public Result HttpError(HttpStatusCode statusCode)
{
var httpError = new HttpError(statusCode)
return Result.Failure(httpError);
}
}
Which clearly and explicitly describes the results.
public Result GetPerson(int id)
{
var person = _database.GetPerson(id);
if (person is null)
return AppError.NotFound();
return Result.Success();
}
Handling Exceptions
Specific overloads have been added to Failure() and Failure<TValue>() to simplify using try-catch blocks and return from them with a result instead of
throwing.
public Result DoSomeWork()
{
try
{
// We must not throw an exception in this method!
}
catch(Exception ex)
{
return Result.Failure(ex);
}
return Result.Success();
}
Static abstract members in interfaces
Note: Applies to .NET 7.0 (C# 11.0) or higher only!
Thanks to the
static abstract members in interfaces
introduced in .NET 7.0 (C# 11.0), it is possible to use generics to obtain access to the methods
of the generic variant of the result. As such the error factory can be enhanced to take advantage of that.
public static AppError
{
public Result NotFound()
{
var notFoundError = new NotFoundError();
return Result.Failure(notFoundError);
}
public TResult NotFound<TResult>()
{
var notFoundError = new NotFoundError();
return TResult.Failure(notFoundError);
}
}
What's new in v9.0
The API for LightResults was completely redesigned for v9.0 to improve performance and remove any potential pits of failure caused by the prior version's use
of properties that could result in exceptions being thrown when values were accessed without checking the state of the result. Thus, there are several breaking
changes, detailed below, that developers must be aware of when upgrading from v8.0 to v9.0.
Breaking changes between v8.0 and v9.0
Factory methods for creating generic results have changed names.
Result.Ok() has been renamed to Result.Success().
Result.Fail() has been renamed to Result.Failure().
Factory methods for creating results with values have changed names and type to allow omitting the type when it is known.
Result<TValue>.Ok() has been renamed and moved to Result.Success<TValue>().
Result<TValue>.Fail() has been renamed to Result.Failure<TValue>().
The Value and Error properties have been removed.
result.Value has been replaced by result.IsSuccess(out var value).
result.Error has been replaced by result.IsError(out var error).
Several constructors of the Error type have been removed or have changed.
Error((string Key, object Value) metadata) has been removed.
Error(IDictionary<string, object> metadata) has been removed.
Error(string message, IDictionary<string, object> metadata) has been changed to
Error(string message, IReadOnlyDictionary<string, object> metadata).
Migrating
The following steps in the following order will reduce the amount of manual work required to migrate and refactor code to use the new API.
Find and replace all instances of Result.Ok( with Result.Success(.
Find and replace all instances of Result.Fail( with Result.Failure(.
Regex find and replace all instances of Result(<[^>]+>)\.Ok\( with Result.Success$1(.
Regex find and replace all instances of Result(<[^>]+>)\.Fail\( with Result.Failure$1(.
Find and replace all instances of .IsSuccess with IsSuccess(out var value).
Find and replace all instances of .IsFailed with IsFailure(out var error).
Find instances of result.Value and refactor them to the use the value exposed by the IsSuccess() method.
Find instances of result.Error and refactor them to the use the error exposed by the IsFailure() method.
New method overloads and property initializers
New overloads have been added for KeyValuePair<string, object> metadata.
Result.Failure(string errorMessage, KeyValuePair<string, object> metadata) has been added.
Result.Failure<TValue>(string errorMessage, KeyValuePair<string, object> metadata) has been added.
New overloads have been added to simplify handling exceptions.
Result.Failure(Exception ex) has been added.
Result.Failure(string errorMessage, Exception ex) has been added.
Result.Failure<TValue>(Exception ex) has been added.
Result.Failure<TValue>(string errorMessage, Exception ex) has been added.
New overloads where added to access the value.
result.IsSuccess(out TValue value) has been added.
result.IsFailure(out IError error, out TValue value) has been added.
New overloads where added to access the first error.
result.IsFailure(out IError error) has been added.
result.IsSuccess(out TValue value, out IError error) has been added.
result.HasError<TError>(out IError error) has been added.
New property initializers where added to Error.
Message { get; } has changed to Message { get; init; }.
Metadata { get; } has changed to Metadata { get; init; }.
