Is there any benefit to this switch / pattern matching idea?

Solution 1:

After trying to do such "functional" things in C# (and even attempting a book on it), I've come to the conclusion that no, with a few exceptions, such things don't help too much.

The main reason is that languages such as F# get a lot of their power from truly supporting these features. Not "you can do it", but "it's simple, it's clear, it's expected".

For instance, in pattern matching, you get the compiler telling you if there's an incomplete match or when another match will never be hit. This is less useful with open ended types, but when matching a discriminated union or tuples, it's very nifty. In F#, you expect people to pattern match, and it instantly makes sense.

The "problem" is that once you start using some functional concepts, it's natural to want to continue. However, leveraging tuples, functions, partial method application and currying, pattern matching, nested functions, generics, monad support, etc. in C# gets very ugly, very quickly. It's fun, and some very smart people have done some very cool things in C#, but actually using it feels heavy.

What I have ended up using often (across-projects) in C#:

• Sequence functions, via extension methods for IEnumerable. Things like ForEach or Process ("Apply"? -- do an action on a sequence item as it's enumerated) fit in because C# syntax supports it well.
• Abstracting common statement patterns. Complicated try/catch/finally blocks or other involved (often heavily generic) code blocks. Extending LINQ-to-SQL fits in here too.
• Tuples, to some extent.

** But do note: The lack of automatic generalization and type inference really hinder the use of even these features. **

All this said, as someone else mentioned, on a small team, for a specific purpose, yes, perhaps they can help if you're stuck with C#. But in my experience, they usually felt like more hassle than they were worth - YMMV.

Some other links:

• Mono.Rocks playground has many similar things (as well as non-functional-programming-but-useful additions).
• Luca Bolognese's functional C# library
• Matthew Podwysocki's functional C# on MSDN

Solution 2:

Arguably the reason that C# doesn't make it simple to switch on type is because it is primarily an object-oriented language, and the 'correct' way to do this in object-oriented terms would be to define a GetRentPrice method on Vehicle and override it in derived classes.

That said, I've spent a bit of time playing with multi-paradigm and functional languages like F# and Haskell which have this type of capability, and I've come across a number of places where it would be useful before (e.g. when you are not writing the types you need to switch on so you cannot implement a virtual method on them) and it's something I'd welcome into the language along with discriminated unions.

[Edit: Removed part about performance as Marc indicated it could be short-circuited]

Another potential problem is a usability one - it's clear from the final call what happens if the match fails to meet any conditions, but what is the behaviour if it matches two or more conditions? Should it throw an exception? Should it return the first or the last match?

A way I tend to use to solve this kind of problem is to use a dictionary field with the type as the key and the lambda as the value, which is pretty terse to construct using object initializer syntax; however, this only accounts for the concrete type and doesn't allow additional predicates so may not be suitable for more complex cases. [Side note - if you look at the output of the C# compiler it frequently converts switch statements to dictionary-based jump tables, so there doesn't appear to be a good reason it couldn't support switching on types]

Solution 3:

In C# 7, you can do:

switch(shape)
{
    case Circle c:
        WriteLine($"circle with radius {c.Radius}");
        break;
    case Rectangle s when (s.Length == s.Height):
        WriteLine($"{s.Length} x {s.Height} square");
        break;
    case Rectangle r:
        WriteLine($"{r.Length} x {r.Height} rectangle");
        break;
    default:
        WriteLine("<unknown shape>");
        break;
    case null:
        throw new ArgumentNullException(nameof(shape));
}