Handling failures and input errors in a functional way

CustomerController.cs

[HttpPost]
public HttpResponseMessage CreateCustomer(string name, string billingInfo)
{
    Result<BillingInfo> billingInfoResult = BillingInfo.Create(billingInfo);
    Result<CustomerName> customerNameResult = CustomerName.Create(name);

    return Result.Combine(billingInfoResult, customerNameResult)
        .OnSuccess(() => _paymentGateway.ChargeCommission(billingInfoResult.Value))
        .OnSuccess(() => new Customer(customerNameResult.Value))
        .OnSuccess(
            customer => _repository.Save(customer)
                .OnFailure(() => _paymentGateway.RollbackLastTransaction())
        )
        .OnSuccess(() => _emailSender.SendGreetings(customerNameResult.Value))
        .OnBoth(result => Log(result))
        .OnBoth(result => CreateResponseMessage(result));
}

Result.cs

public class Result
{
    public bool Success { get; private set; }
    public string Error { get; private set; }

    public bool Failure
    {
        get { return !Success; }
    }

    protected Result(bool success, string error)
    {
        Contracts.Require(success || !string.IsNullOrEmpty(error));
        Contracts.Require(!success || string.IsNullOrEmpty(error));

        Success = success;
        Error = error;
    }

    public static Result Fail(string message)
    {
        return new Result(false, message);
    }

    public static Result<T> Fail<T>(string message)
    {
        return new Result<T>(default(T), false, message);
    }

    public static Result Ok()
    {
        return new Result(true, String.Empty);
    }

    public static Result<T> Ok<T>(T value)
    {
        return new Result<T>(value, true, String.Empty);
    }

    public static Result Combine(params Result[] results)
    {
        foreach (Result result in results)
        {
            if (result.Failure)
                return result;
        }

        return Ok();
    }
}


public class Result<T> : Result
{
    private T _value;

    public T Value
    {
        get
        {
            Contracts.Require(Success);

            return _value;
        }
        [param: AllowNull]
        private set { _value = value; }
    }

    protected internal Result([AllowNull] T value, bool success, string error)
        : base(success, error)
    {
        Contracts.Require(value != null || !success);

        Value = value;
    }
}

ResultExtensions.cs

public static class ResultExtensions
{
    public static Result OnSuccess(this Result result, Func<Result> func)
    {
        if (result.Failure)
            return result;

        return func();
    }

    public static Result OnSuccess(this Result result, Action action)
    {
        if (result.Failure)
            return result;

        action();

        return Result.Ok();
    }

    public static Result OnSuccess<T>(this Result<T> result, Action<T> action)
    {
        if (result.Failure)
            return result;

        action(result.Value);

        return Result.Ok();
    }

    public static Result<T> OnSuccess<T>(this Result result, Func<T> func)
    {
        if (result.Failure)
            return Result.Fail<T>(result.Error);

        return Result.Ok(func());
    }

    public static Result<T> OnSuccess<T>(this Result result, Func<Result<T>> func)
    {
        if (result.Failure)
            return Result.Fail<T>(result.Error);

        return func();
    }

    public static Result OnSuccess<T>(this Result<T> result, Func<T, Result> func)
    {
        if (result.Failure)
            return result;

        return func(result.Value);
    }

    public static Result OnFailure(this Result result, Action action)
    {
        if (result.Failure)
        {
            action();
        }

        return result;
    }

    public static Result OnBoth(this Result result, Action<Result> action)
    {
        action(result);

        return result;
    }

    public static T OnBoth<T>(this Result result, Func<Result, T> func)
    {
        return func(result)
    }
}

The Contracts class where you put all your common validate there?

You are missing a semicolon on line 74 in ResultExtensions.

Hey @vkhorikov thank you! I'm using .Net core and the [param: AllowNull] doesn't compile , I'm getting

the type or namespace name 'AllowNullAttribute' could not be found...

Any suggestion?

@ShaharShokrani you need to install NullGuard.Fody for that. It's optional, though, this would work without it. More on the null guard:

https://www.nuget.org/packages/NullGuard.Fody/
https://enterprisecraftsmanship.com/2015/03/13/functional-c-non-nullable-reference-types/

What is Contracts?

It's a simple helper, e.g

public static class Guard
{
    public static void Require(bool precondition, string exceptionMessage)
    {
        if (!precondition)
            throw new CodeContractException(exceptionMessage);
    }
}