Implicit return for Result does not allow Interface<T> - must be concrete type ie List<T> not IEnumerable<T>

[martinbryant]

Example

public Result<Thing, string> GetThing()
{
        bool success = false;
        var thing = new Thing();
        if(!success)
            return "";
    
        return thing;
}

Compiles

public Result<IEnumerable<Thing>, string> GetThingsList()
{
    bool success = false;
    List<Thing> things = new List<Thing>() { new Thing() };
    if(!success)
        return "";

    return things;
}

Compiles

public Result<IEnumerable<Thing>, string> GetThingsIEnumerable()
{
    bool success = false;
    IEnumerable<Thing> things = new List<Thing>() { new Thing() };
    if(!success)
        return "";

    return things;
}

Does not compile - Cannot implicitly convert type 'System.Collections.Generic.IEnumerable' to 'CSharpFunctionalExtensions.Result<System.Collections.Generic.IEnumerable, string>'

It seems as if it does not work correctly for anything that is an Interface ie IEnumerable, IList, ICollection.

Workaround

public Result<IEnumerable<Thing>, string> GetThingsIEnumerable()
{
    bool success = false;
    IEnumerable<Thing> things = new List<Thing>() { new Thing() };
    if(!success)
        return "";

    return Result.Success<IEnumerable<Thing>, string>(things);
}

[hankovich]

Yes, it's a C# limitation, it has no elegant solutons/workarounds. We usually return List<Thing> via interface reference (as in your second example).

Additional info:
https://stackoverflow.com/a/143567
dotnet/roslyn#14186

[martinbryant]

Ok @hankovich I appreciate you confirming that and can understand the reasoning for it.