User-defined conversion operator from base class

Question

Introduction

I am aware that \"user-defined conversions to or from a base class are not allowed\". MSDN gives, as an explanation to this rule, \"You

Answer 1

(Invoking necromancy protocols...)

Here's my use-case:

class ParseResult
{
    public static ParseResult Error(string message);
    public static ParseResult Parsed(T value);

    public bool IsError { get; }
    public string ErrorMessage { get; }
    public IEnumerable WarningMessages { get; }

    public void AddWarning(string message);
}

class ParseResult : ParseResult
{
    public static implicit operator ParseResult(ParseResult result); // Fails
    public T Value { get; }
}

...

ParseResult ParseSomeBigLongTypeName()
{
    if (SomethingIsBad)
        return ParseResult.Error("something is bad");
    return ParseResult.Parsed(new SomeBigLongTypeName());
}

Here Parsed() can infer T from it's parameter, but Error can't, but it can return a typeless ParseResult that is convertible to ParseResult - or it would be if not for this error. The fix is to return and convert from a subtype:

class ParseResult
{
    public static ErrorParseResult Error(string message);
    ...
}

class ErrorParseResult : ParseResult {}

class ParseResult
{
    public static implicit operator ParseResult(ErrorParseResult result);
    ...
}

And everything is happy!