ResultHelper.MergeWithValue enumerates results parameter multiple times

[mtyski]

Extension method IEnumerable<Result<TValue>>.Merge() enumerates passed results collection multiple times. This might cause side effects to be introduced multiple times; the following code sample throws an InvalidOperationException:

using FluentResults;

var sth = new Something(
    Enumerable.Range(1, 5)
        .ToHashSet());

var result = Enumerable.Range(6, 2)
    .Select(number => sth.AddNumber(number))
    .Merge(); // Exception is thrown here;

Console.WriteLine($"Merger is a {(result.IsSuccess ? "success" : "failure")}");

internal class Something
{
    private readonly HashSet<int> numbers;

    public Something(HashSet<int> numbers)
    {
        this.numbers = numbers;
    }

    public Result<Something> AddNumber(int number) =>
        Result.OkIf(numbers.Add(number), "Can't add a duplicate!")
            .ToResult(this);
}

The workaround is to materialize the results enumerable before merging. Fixing that requires change in the implementation of MergeWithValue to one that enumerates the collection once; the simplest (and the most dumbed-down) refactor would resemble this:

public static Result<IEnumerable<TValue>> MergeWithValue<TValue>(IEnumerable<Result<TValue>> results)
{
    var values = new List<TValue>();
    var finalResult = Result.Ok<IEnumerable<TValue>>(Array.Empty<TValue>());

    foreach (var result in results)
    {
        finalResult.Reasons.AddRange(result.Reasons);
        if (result.IsSuccess)
        {
            values.Add(result.Value);
        }
    }

    return finalResult.IsSuccess ? finalResult.WithValue(values) : finalResult;
}

Alternatively, results can be materialized first and then the rest can remain mostly unchanged:

public static Result<IEnumerable<TValue>> MergeWithValue<TValue>(IEnumerable<Result<TValue>> results)
{
    var resultList = results.ToList();

    var finalResult = Result.Ok<IEnumerable<TValue>>(Array.Empty<TValue>())
        .WithReasons(resultList.SelectMany(result => result.Reasons));

    return finalResult.IsSuccess
        ? finalResult.WithValue(
            resultList.Select(r => r.Value)
                .ToList())
        : finalResult;
}

[altmann]

Thanks for your feedback. I will have a look at it.

[altmann]

Thanks again for creating the issue. The new fixed version will be released in the next hour... https://www.nuget.org/packages/FluentResults/3.13.0