A method group of one

I'm implementing the semantic analysis of dynamic expressions in Roslyn this week, so I'm fielding a lot of questions within the team on the design of the dynamic feature of C# 4. A question I get fairly frequently in this space is as follows:

public class Alpha
  public int Foo(string x) { ... }
dynamic d = whatever;
Alpha alpha = MakeAlpha();
var result = alpha.Foo(d);

How is this analyzed? More specifically, what's the type of local result?

If the receiver (that is, alpha) of the call were of type dynamic then there would be little we could do at compile time. We'd analyze the compile-time types of the arguments and emit a dynamic call site that caused the semantic analysis to be performed at runtime, using the runtime type of the dynamic expression. But that's not the case here. We know at compile time what the type of the receiver is. One of the design principles of the C# dynamic feature is that if we have a type that is known at compile time, then at runtime the type analysis honours that. In other words, we only use the runtime type of the things that were actually dynamic; everything else we use the compile-time type. If MakeAlpha() returns a derived class of Alpha, and that derived class has more overloads of Foo, we don't care.

Because we know that we're going to be doing overload resolution on a method called Foo on an instance of type Alpha, we can do a "sanity check" at compile time to determine if we know that for sure, this is going to fail at runtime. So we do overload resolution, but instead of doing the full overload resolution algorithm (eliminate inapplicable candidates, determine the unique best applicable candidate, perform final validation of that candidate), we do a partial overload resolution algorithm. We get as far as eliminating the inapplicable candidates, and if that leaves one or more candidates then the call is bound dynamically. If it leaves zero candidates then we report an error at compile time, because we know that nothing is going to work at runtime.

Now, a seemingly reasonable question to ask at this point is: overload resolution in this case could determine that there is exactly one applicable candidate in the method group, and therefore we can determine statically that the type of result is int, so why do we instead say that the type of result is dynamic?

That appears to be a reasonable question, but think about it a bit more. If you and I and the compiler know that overload resolution is going to choose a particular method then why are we making a dynamic call in the first place? Why haven't we cast d to string? This situation is rare, unlikely, and has an easy workaround by inserting casts appropriately (either casting the call expression to int or the argument to string). Situations that are rare, unlikely and easily worked around are poor candidates for compiler optimizations. You asked for a dynamic call, so you're going to get a dynamic call.

That's reason enough to not do the proposed feature, but let's think about it a bit more deeply by exploring a variation on this scenario that I glossed over above. Eta Corporation produces:

public class Eta {}

and Zeta Corporation extends this code:

public class Zeta : Eta
  public int Foo(string x){ ... }
dynamic d = whatever;
Zeta zeta = new Zeta();
var result = zeta.Foo(d);

Suppose we say that the type of result is int because the method group has only one member. Now suppose that in the next version, Eta Corporation supplies a new method:

public class Eta
  public string Foo(double x){...}

Zeta corporation recompiles their code, and hey presto, suddenly result is of type dynamic! Why should Eta Corporation's change to the base class cause the semantic analysis of code that uses a derived class to change? This seems unexpected. C# has been carefully designed to avoid these sorts of "Brittle Base Class" failures; see my other articles on that subject for examples of how we do that.

We can make a bad situation even worse. Suppose Eta's change is instead:

public class Eta
  protected string Foo(double x){...}

Now what happens? Should we say that the type of result is int when the code appears outside of class Zeta, because overload resolution produces a single applicable candidate, but dynamic when it appears inside, because overload resolution produces two such candidates? That would be quite bizarre indeed.

The proposal is simply too much cleverness in pursuit of too little value. We've been asked to perform a dynamic binding, and so we're going to perform a dynamic binding; the result should in turn be dynamic. The benefits of being able to statically deduce types of dynamic expressions does not pay for the costs, so we don't attempt to do so. If you want static analysis then don't turn it off in the first place.

Next time: the dynamic taint of method type inference.

Comments (8)
  1. Kyle says:

    Man I love reading your blog.

  2. Scott Kelley says:

    "I'm implementing the semantic analysis of dynamic expressions in Roslyn this week"

    That statement is stunningly mundane coming from you, from anyone else it would invite ridicule. Keep setting the bar high, good sir.

    Thank you,


  3. Motti says:

    > This situation is rare, unlikely, and has an easy workaround by inserting casts appropriately

    I don't know how rare it is, I ran into it twice this week, in fact it prompted me to write a blog post (which I scheduled for next month but will post now for purposes of this comment).


    That is really good feedback. Thanks for commenting! — Eric

  4. martinien kouakou says:

    «Je suis la mise en œuvre de l'analyse sémantique des expressions dynamiques à Roslyn cette semaine"

    Cette déclaration est incroyablement banal venant de vous, de toute autre personne qu'il inviterait ridicule. Gardez placer la barre haute, mon bon monsieur.

    Je vous remercie, martinien

  5. Stuart says:

    Just to chime in and agree with the point about the unfortunate interaction between var and dynamic. Obviously you can't introduce a new default warning now without breaking back compat, but it'd be awesome if there was some way to make it a compiletime warning anytime a "var" resolves to "dynamic".

  6. raceprouk@hotmail.co.uk says:

    @Stuart: An interesting idea, but someone may actually want a var to resolve to dynamic e.g. as the result of a LINQ statement/method chain. In that instance, the warning would actually hinder the developer, especially as some developers turn on 'Treat warnings as errors' (as I do in personal projects).

  7. CarlD says:

    @RaceProUK in that case, I'd suggest the programmer use the dynamic keyword instead of var.  I think it's clearer that way anyway – the silent conversion of an explicit type into dynamic in these circumstances is something that ought to require more deliberate action on the part of the programmer.  I'd hazard a guess that vast majority of cases in the wild where var was bound to dynamic are in fact unintended by the programmer.

  8. Scott Slack says:

    At what point in the language evolution do you realize you've made a mistake, but can not reverse out without breaking too much code?

    If we realize that we've made a design mistake before we ship the product to customers then we can fix it. If we've shipped it, we live with it forever. Pretty much everyone agrees that it's deeply unfortunate that there are two incompatible syntaxes for lambda expressions/anonymous methods, but we cannot eliminate anonymous method syntax; it would break too much code. — Eric

    Could nobody see the potential impact of type inference with a chunk of dynamic and a twist of generics?

    I don't understand the question. Obviously we did see exactly how dynamic code would have a tricky interaction with generic method type inference, and designed the language so that it handled it in a reasonable manner. This blog post is about how we decided to do so. — Eric

    Isn't this how you make Red Matter?

    I have no idea what you're talking about. — Eric

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