Making the world a better place, one line of code at a time

Code Snippets: Impossible Code

Posted on Tuesday, June 30, 2015

This post is part of a series of blog posts called code snippets. These blog posts will explore successively more interesting ways to do simple tasks or abuse language features.

I recently set out to create snippets of code that have nothing inherently wrong with them, but can never appear in a valid program.

Impossible accessibility

The simplest example is a statement that uses internal types from two different assemblies, so that there is no project that it could legally appear in:

// In A.dll:
public class Base {
    internal static void Method() { }

// In B.dll:
public class Container {
    public class Derived : Base { }


Note that the method itself is not impossible to call; Base.Method() is perfectly legal anywhere in the first assembly. However, the specific expression Container.Derived.Method() is impossible, since it uses types from both assemblies.

Adding an [InternalsVisibleTo] attribute to the first assembly, or compiling both assemblies with circular references (this is possible, if you try very hard), would make this statement legal. To prevent that, you can simply make both nested members protected.

void expressions

A more interesting approach is to create an expression of type void. This expression is completely fine, except that there is no way to use an expression of type void anywhere.

The only valid place that the type void can appear in C# is as a method return type; thus, the only way to make an expression of type void is a method call. Method calls are legal as expression statements, so that is perfectly fine.

However, there is one feature in the language that lets you call an expression with an arbitrary return type, in a syntax which is not legal as an expression statement: LINQ queries.

class Impossible {
    public static void Select(Func<int, int> a) { }

from x in Impossible select 1;

This would compile to Impossible.Select(x => 1), which is a perfectly fine expression statement, returning void. However, query expressions, unlike method call expressions, are not valid as expression statements, so this call is illegal.

Any attempt to use this query will result in either “CS0201: Only assignment, call, increment, decrement, and new object expressions can be used as a statement”, “CS0127 Since ‘Method()’ returns void, a return keyword must not be followed by an object expression”, or “CS0815: Cannot assign void to an implicitly-typed variable”. (except for a bug in the RC release of the Roslyn compiler)

Side note: I need to take the unusual approach of making a LINQ query against a static method on a type to prevent people from making it compile by adding a matching extension method that does not return void.

Impossible overload resolution

A subtler approach is to make two overloads of a generic method such that it is impossible to distinguish between them:

void Method<T, U>(T p) { }
void Method<T, U>(U p) { }

There is no way to call either of these methods with the same type for both generic type parameters (eg, Method<int, int>).

Ordinarily, C# has a number of ways to help disambiguate between generic overloads. You can cast ambiguous parameters to the exact argument type of the desired overload. You can explicitly specify type parameters. You can call the method through a specific qualifier (to disambiguate between static and instance methods, or between methods inherited from a base class). You can even pass named parameters if the two overloads have different parameters names.

However, these methods are identical in all of these regards, leaving no way to tell the compiler which one you want to call. (note that it is possible to call them at runtime using Reflection)

Generic constraint conflict

Another option is to create a method with conflicting generic type constraints, such that there is no possible type that will satisfy the constraints. Ordinarily, the compiler will not let you do that; a type parameter cannot be constrained to two different classes. However, you can bypass this restriction using more generics:

class Container<T, U> {
	public void Method<Z>() where Z : T, U {}

It is impossible to call Method on any instance parameterized with classes that do not inherit one another (eg, Container<Exception, Type>). Unlike the previous example, it is completely impossible to call this method on such an instantiation, even with reflection – there is no type that is valid value for Z.

Impossible inheritance

Any of these impossible methods can also be used to make a class that can never have any instances, by making an abstract method that cannot be implemented:

abstract class Base<T, U> {
    public abstract void Method(T x);
    public abstract void Method(U x);
abstract class Impossible : Base<int, int> { }

Any attempt to create a concrete non-abstract subclass of Impossible will result in “CS0462 The inherited members Base<T, U>.Method(T) and Base<T, U>.Method(U) have the same signature in type ‘Impossible’, so they cannot be overridden”.

abstract class Base<T, U> {
    public abstract void Method<Z>() where Z : T, U;
abstract class Impossible : Base<Type, Exception> { }

Here, implementing Impossible will give “CS0455 Type parameter ‘Z’ inherits conflicting constraints ‘Exception’ and ‘Type’”.

Interface method conflict

Since Java is less feature-rich than C#, there are far fewer examples of impossible code in Java. However, this feature sparsity can be used to create code which is only impossible in Java – inheriting conflicting interface methods:

interface A {
    int method();
interface B {
    long method();
final class C implements A, B { }

In Java, it is impossible to implement C. In C#, this would be possible with explicit interface implementation, which essentially implements each method with a private (and therefore unique) compiler-generated name, avoiding problems of identical overloads. However, since Java does not support this feature, you’d need to make two methods with the same signature but different return types, which is impossible.

Side note: I need to add final because otherwise, making C abstract would let it compile without implementing the method.

If you can think of other examples of impossible code, please comment!

Categories: C#, Java, code-snippets Tweet this post

comments powered by Disqus