Is it possible to compile a C# method that has non-void return type, but does not return a value?

Question

Is it possible to somehow compile XM() { /*...*/ } given the following constraints:

1. You can't add any other modifiers to M .
2. /*...*/ does not contain return or throw , and you can't change it.
3. You can't add rewriters to the compiler pipeline.
4. You can change everything else around M .

It might seem like a nonsensical question, but it is actually valuable for DSLs.
Eg I could define something like task M() {} which would just denote that it is a task, and have nothing to do with a return value.

Answer 1

Adding in some additional information from the C# specification.

This first paragraph states that a method must have a return type, or void.

Methods have a (possibly empty) list of parameters, which represent values or variable references passed to the method, and a return type, which specifies the type of the value computed and returned by the method. A method's return type is void if it does not return a value.

This second paragraph specifies that a method with a non-void return type must have a calculable return expression.

A method can use return statements to return control to its caller. In a method returning void, return statements cannot specify an expression. In a method returning non-void, return statements must include an expression that computes the return value.

Even though the specification does not explicitly state that method must contain a return statement, I believe it implicitly says so.

So no, it is not possible according to the C# specification.

Answer 2

So you have a method:

public Foo Bar()
{
    DoX();

    DoY();

    DoZ();
}

Which doesn't compile, because the only path in the method doesn't return anything.

Given your constraints (basically: you have to compile the code as-is, with a standard C# compiler), the answer is: no, you can't make this compile.

Answer 3

maybe,

abstract class Whatever
{
    public abstract X M<X>();
}