does C# support sum types? GP was talking about C# specifically. Agree with your analysis though: it's essentially a partial function, so the range (codomain)

does C# support sum types? GP was talking about C# specifically. Agree with your analysis though: it's essentially a partial function, so the range (codomain) of the function includes "no defined output for the specified input". It's therefore not a bug to return "not found". An algebraic datatype like Either/Optional/Result allows that to be encoded in the type system, and so provides explicit support syntactically and semantically for handling the situation. I don't think C# supports sum types (though may well be wrong there, not so close to the language). Assuming not, the question is how to achieve the desired outcomes. Those might reasonably be one or more of: * minimise the risk of null de-reference * maintain performance * be idiomatic * be understandable * be explicit If (and it's a big if) the only language options are nulls and exceptions, there's not a clear winner there. So exploring the tradeoffs does seem like a reasonable discussion.