In our current project, we have a very simplified version of a Result object. One of the things we are interested in adding to it is the ability to take multiple results containing an array of items and then combining them into a single result object containing all of the items from the other results.

The issue we are running into is when trying to make this generic through the IEnumerable<T> interface, we get an error stating Cannot convert Result<string[]> to Result<IEnumerable<string>>.

Below is a simple reproduction of the error. I know I could just add a method to handle T[], but is that the only option? I’m guessing this is something to do with covariance/contravariance, but I have never been good at knowing which is which and when to use what.

class Result
{
    public static Result<T> Create<T>(T value) => new Result<T>(value);
    public static Result<IEnumerable<T>> Flatten<T>(params Result<IEnumerable<T>>[] results)
    {
        return new Result<IEnumerable<T>>(results.SelectMany(x => x.Value));
    }
}

class Result<T>
{
    internal Result(T value) => Value = value;

    public T Value { get; internal set; }
}

var result1 = Result.Create<string[]>(["A", "B"]);
var result2 = Result.Create<string[]>(["C", "D"]);
var result3 = Result.Create<string[]>(["E", "F"]);

// Should create a result of ["A", "B", "C", "D", "E", "F"]
var result = Result.Flatten<string>(result1, result2, result3);