FluentResults

FluentResults is a lightweight .NET library developed to solve a common problem. It returns an object indicating success or failure of an operation instead of throwing/using exceptions.

You can install FluentResults with NuGet:

Install-Package FluentResults

Key Features

Generalised container which works in all contexts (ASP.NET MVC/WebApi, WPF, DDD Domain Model, etc)
Store multiple errors in one Result object
Store powerful and elaborative Error and Success objects instead of only error messages in string format
Designing Errors/Success in an object-oriented way
Store the root cause with chain of errors in a hierarchical way
Provide
- .NET Standard, .NET Core and .NET Full Framework support
- SourceLink support
- powerful code samples which show the integration with famous or common frameworks/libraries
FluentAssertions Extension to assert FluentResult objects in an elegant way

Why Results instead of exceptions

To be honest, the pattern - returning a Result object indicating success or failure - is not at all a new idea. This pattern comes from functional programming languages. With FluentResults this pattern can also be applied in .NET/C#.

The article Exceptions for Flow Control by Vladimir Khorikov describes very good in which scenarios the Result pattern makes sense and in which not. See the list of Best Practices and the list of resources to learn more about the Result Pattern.

Creating a Result

A Result can store multiple Error and Success messages.

// create a result which indicates success
Result successResult1 = Result.Ok();

// create a result which indicates failure
Result errorResult1 = Result.Fail("My error message");
Result errorResult2 = Result.Fail(new StartDateIsAfterEndDateError(startDate, endDate));

The class Result is typically used by void methods which have no return value.

public Result DoTask()
{
    if (this.State == TaskState.Done)
        return Result.Fail("Task is in the wrong state.");

    // rest of the logic

    return Result.Ok();
}

Additionally a value from a specific type can also be stored if necessary.

// create a result which indicates success
Result<int> successResult1 = Result.Ok(42);
Result<MyCustomObject> successResult2 = Result.Ok(new MyCustomObject());

// create a result which indicates failure
Result<int> errorResult = Result.Fail<int>("My error message");

The class Result<T> is typically used by methods with a return type.

public Result<Task> GetTask()
{
    if (this.State == TaskState.Deleted)
        return Result.Fail<Task>("Deleted Tasks can not be displayed.");

    // rest of the logic

    return Result.Ok(task);
}

Processing a Result

After you get a Result object from a method you have to process it. This means, you have to check if the operation was completed successfully or not. The properties IsSuccess and IsFailed in the Result object indicate success or failure. The value of a Result<T> can be accessed via the properties Value and ValueOrDefault.

Result<int> result = DoSomething();
     
// get all reasons why result object indicates success or failure. 
// contains Error and Success messages
IEnumerable<Reason> reasons = result.Reasons;

// get all Error messages
IEnumerable<Error> errors = result.Errors;

// get all Success messages
IEnumerable<Success> successes = result.Successes;

if (result.IsFailed)
{
    // handle error case
    var value1 = result.Value; // throws exception because result is in failed state
    var value2 = result.ValueOrDefault; // return default value (=0) because result is in failed state
    return;
}

// handle success case
var value3 = result.Value; // return value and doesn't throw exception because result is in success state
var value4 = result.ValueOrDefault; // return value because result is in success state

Designing errors and success messages

There are many Result Libraries which store only simple string messages. FluentResults instead stores powerful object-oriented Error and Success objects. The advantage is all the relevant information of an error is encapsulated within one class.

The classes Success and Error inherit from the base class Reason. If at least one Error object exists in the Reasons property then the result indicates a failure and the property IsSuccess is false.

You can create your own Success or Error classes when you inherit from Success or Error.

public class StartDateIsAfterEndDateError : Error
{
    public StartDateIsAfterEndDateError(DateTime startDate, DateTime endDate)
        : base($"The start date {startDate} is after the end date {endDate}")
    { 
        Metadata.Add("ErrorCode", "12");
    }
}

With this mechanism you can also create a class Warning. You can choose if a Warning in your system indicates a success or a failure by inheriting from Success or Error classes.

Further features

Chaining error and success messages

In some cases it is necessary to chain multiple error and success messages in one result object.

var result = Result.Fail("error message 1")
                   .WithError("error message 2")
                   .WithError("error message 3")
                   .WithSuccess("success message 1");

Create a result depending on success/failure condition

Very often you have to create a fail or success result depending on a condition. Usually you can write it in this way:

var result = string.IsNullOrEmpty(firstName) ? Result.Fail("First Name is empty") : Result.Ok();

With the methods FailIf() and OkIf() you can also write in a more readable way:

var result = Result.FailIf(string.IsNullOrEmpty(firstName), "First Name is empty");

Try

In some scenarios you want to execute an action. If this action throws an exception then the exception should be catched and transformed to a result object.

var result = Result.Try(() => DoSomethingCritical());

In the above example the default catchHandler is used. The behavior of the default catchHandler can be overwritten via the global Result settings (see next example). You can control how the Error object looks.

Result.Setup(cfg =>
{
    cfg.DefaultTryCatchHandler = exception =>
    {
        if (exception is SqlTypeException sqlException)
            return new ExceptionalError("Sql Fehler", sqlException);

        if (exception is DomainException domainException)
            return new Error("Domain Fehler")
                .CausedBy(new ExceptionError(domainException.Message, domainException));

        return new Error(exception.Message);
    };
});

var result = Result.Try(() => DoSomethingCritical());

It is also possible to pass a custom catchHandler via the Try(..) method.

var result = Result.Try(() => DoSomethingCritical(), ex => new MyCustomExceptionError(ex));

Root cause of the error

You can also store the root cause of the error in the error object. With the method CausedBy(...) the root cause can be passed as Error, list of Errors, string, list of strings or as exception. The root cause is stored in the Reasons property of the error object.

Example 1 - root cause is an exception

try
{
    //export csv file
}
catch(CsvExportException ex)
{
    return Result.Fail(new Error("CSV Export not executed successfully").CausedBy(ex));
}

Example 2 - root cause is an error

Error rootCauseError = new Error("This is the root cause of the error");
Result result = Result.Fail(new Error("Do something failed", rootCauseError));

Example 3 - reading root cause from errors

Result result = ....;
if (result.IsSuccess)
   return;

foreach(var error in result.Errors)
{
    foreach(var causedByExceptionalError in error.Reasons.OfType<ExceptionalError>())
    {
        Console.WriteLine(causedByExceptionalError.Exception);
    }
}

Metadata

It is possible to add metadata to Error or Success objects.

One way of doing that is to call the method WithMetadata(...) directly where result object is being created.

var result1 = Result.Fail(new Error("Error 1").WithMetadata("metadata name", "metadata value"));

var result2 = Result.Ok()
                    .WithSuccess(new Success("Success 1")
                                 .WithMetadata("metadata name", "metadata value"));

Another way is to call WithMetadata(...) in constructor of the Error or Success class.

public class DomainError : Error
{
    public DomainError(string message)
        : base(message)
    { 
        WithMetadata("ErrorCode", "12");
    }
}

Merging

Multiple results can be merged with the static method Merge().

var result1 = Result.Ok();
var result2 = Result.Fail("first error");
var result3 = Result.Ok<int>();

var mergedResult = Result.Merge(result1, result2, result3);

Converting

A result object can be converted to another result object with methods ToResult() and ToResult<TValue>().

// converting a result to a result from type Result<int> with default value of int
Result.Ok().ToResult<int>();

// converting a result to a result from type Result<int> with a custom value
Result.Ok().ToResult<int>(5);

// converting a failed result to a result from type Result<int> without passing a custom value
// because result is in failed state and therefore no value is needed
Result.Fail("Failed").ToResult<int>();

// converting a result to a result from type Result<float>
Result.Ok<int>(5).ToResult<float>(v => v);

// converting a result from type Result<int> to result from type Result<float> without passing the converting
// logic because result is in failed state and therefore no converting logic needed
Result.Fail<int>("Failed").ToResult<float>();

// converting a result to a result from type Result
Result.Ok<int>().ToResult();

Handling/catching errors

Similar to the catch block for exceptions, the checking and handling of errors within Result object is also supported using some methods:

// check if the Result object contains an error from a specific type
result.HasError<MyCustomError>();

// check if the Result object contains an error from a specific type and with a specific condition
result.HasError<MyCustomError>(myCustomError => myCustomError.MyField == 2);

// check if the Result object contains an error with a specific metadata key
result.HasError(error => error.HasMetadataKey("MyKey"));

// check if the Result object contains an error with a specific metadata
result.HasError(error => error.HasMetadata("MyKey", metadataValue => (string)metadataValue == "MyValue"));

Handling successes

Checking if a result object contains a specific success object can be done with the method HasSuccess()

// check if the Result object contains a success from a specific type
result.HasSuccess<MyCustomSuccess>();

// check if the Result object contains a success from a specific type and with a specific condition
result.HasSuccess<MyCustomSuccess>(success => success.MyField == 3);

Logging

Sometimes it is necessary to log results. First create a logger.

public class MyConsoleLogger : IResultLogger
{
    public void Log(string context, ResultBase result)
    {
        Console.WriteLine("{0}", result);
    }
}

Then you have to register your logger.

var myLogger = new MyConsoleLogger();
Result.Setup(cfg => {
    cfg.Logger = myLogger;
});

Finally the logger can be used.

var result = Result.Fail("Operation failed")
    .Log();

Additionally, a context can be passed in form of a string.

var result = Result.Fail("Operation failed")
    .Log("logger context");

Asserting FluentResult objects

Try it with the power of FluentAssertions and FluentResults.Extensions.FluentAssertions

Samples/Best Practices

Here are some samples and best practices to be followed while using FluentResult or the Result pattern in general with some famous or commonly used frameworks and libraries.

Powerful domain model inspired by Domain Driven Design

Domain model with a command handler
Protecting domain invariants by using for example factory methods returning a Result object
Make each error unique by making your own custom Error classes inheriting from Error class
If the method doesn't have a failure scenario then don't use the Result class as return type
Be aware that you can merge multiple failed results or return the first failed result asap

Serializing Result objects (ASP.NET WebApi, Hangfire)

Asp.net WebController
Hangfire Job
Don't serialize FluentResult result objects.
Make your own custom ResultDto class for your public api in your system boundaries
- So you can control which data is submitted and which data is serialized
- Your public api is independent of third party libraries like FluentResults
- You can keep your public api stable

MediatR request handlers returning Result objects

Full functional .NET Core sample code with commands/queries and a ValidationPipelineBehavior
Returns business validation errors via a Result object from a MediatR request handler back to the consumer
Don't throw exceptions based on business validation errors
Inject command and query validation via MediatR PipelineBehavior and return a Result object instead of throwing an exception

Interesting Resources about Result Pattern

Contributors

Thanks to all the contributers and to all the people who gave feedback!

yanjustino / FluentResults