What is the purpose of hidebysig in a MSIL method?

Question

Using ildasm and a C# program e.g.

static void Main(string[] args)
{

}

gives:

.method private hidebysig static void  Main(string[] args) cil managed
{
  .entrypoint
  // Code size       2 (0x2)
  .maxstack  8
  IL_0000:  nop
  IL_0001:  ret
} // end of method Program::Main

What does the hidebysig construct do?

Answer 1

From ECMA 335, section 8.10.4 of partition 1:

The CTS provides independent control over both the names that are visible from a base type (hiding) and the sharing of layout slots in the derived class (overriding). Hiding is controlled by marking a member in the derived class as either hide by name or hide by name-and-signature. Hiding is always performed based on the kind of member, that is, derived field names can hide base field names, but not method names, property names, or event names. If a derived member is marked hide by name, then members of the same kind in the base class with the same name are not visible in the derived class; if the member is marked hide by name-and-signature then only a member of the same kind with exactly the same name and type (for fields) or method signature (for methods) is hidden from the derived class. Implementation of the distinction between these two forms of hiding is provided entirely by source language compilers and the reflection library; it has no direct impact on the VES itself.

(It's not immediately clear from that, but hidebysig means "hide by name-and-signature".)

Also in section 15.4.2.2 of partition 2:

hidebysig is supplied for the use of tools and is ignored by the VES. It specifies that the declared method hides all methods of the base class types that have a matching method signature; when omitted, the method should hide all methods of the same name, regardless of the signature.

As an example, suppose you have:

public class Base
{
    public void Bar()
    {
    }
}

public class Derived : Base
{
    public void Bar(string x)
    {
    }
}

...

Derived d = new Derived();
d.Bar();

That's valid, because Bar(string) doesn't hide Bar(), because the C# compiler uses hidebysig. If it used "hide by name" semantics, you wouldn't be able to call Bar() at all on a reference of type Derived, although you could still cast it to Base and call it that way.

EDIT: I've just tried this by compiling the above code to a DLL, ildasming it, removing hidebysig for Bar() and Bar(string), ilasming it again, then trying to call Bar() from other code:

Derived d = new Derived();
d.Bar();

Test.cs(6,9): error CS1501: No overload for method 'Bar' takes '0' arguments

However:

Base d = new Derived();
d.Bar();

(No compilation problems.)