Lazy error evaluation of Result.OkIf and Result.FailIf

[mtyski]

As far as I am aware, the factory methods OkIf and FailIf need the Error instances to be provided at the moment they are called.

This may pop up in scenarios, where a reported Error instance requires access to the data which might not exist:

var list = Enumerable.Range(1, 9).ToList();

// will throw an InvalidOperationException
var result = Result.FailIf(
    list.Any(IsDivisibleByTen),
    $"Item {list.First(IsDivisibleByTen)} should not be on the list!");

// snip

static bool IsDivisibleByTen(int i) => i % 10 == 0;

The proposed feature would be to implement following method overloads:

• Result.OkIf(bool condition, Func<string>)
• Result.OkIf(bool condition, Func<Error>)
• Result.FailIf(bool, Func<string>)
• Result.FailIf(bool, Func<Error>)

which would enable lazy evaluation of the errors; modifying the previous snippet:

var list = Enumerable.Range(1, 9).ToList();

// will not throw, as the Func<string> should not be called
var result = Result.FailIf(
    list.Any(IsDivisibleByTen),
    () => $"Item {list.First(IsDivisibleByTen)} should not be on the list!");

// snip

static bool IsDivisibleByTen(int i) => i % 10 == 0;

The workarounds to this are:

• use safe methods, which do not throw exceptions in such cases or
• use ternary / if statements.

[altmann]

Looks good. Would be happy if you create a pr for this feature. I would be more happy if you also add some tests and extend the readme file with this feature. I will encrease the version number, write the changelog and generate the package.

Thanks in advance

[mtyski]

Hi @altmann, thanks for a quick reply! The pull request, including overloads implementation, unit tests and a short section in the README is opened and ready for review.

[altmann]

thank you for the great pr. I merged it already into the master branch. New package will be generated hopefully tomorrow.

[altmann]

New package is out: https://www.nuget.org/packages/FluentResults