method ref to arbitrary value type instance

From: Andrew (anonymous_at_discussions.microsoft.com)
Date: 02/13/04 

Date: Thu, 12 Feb 2004 17:21:04 -0800

It seems that in C#, given an instance "a" of some value type "A", one can pass a by reference to functions with signatures of either "void f(object obj)" or "void g(ref A obj)". But passing a into a function with signature "void f(ref object obj)" gives a compile error -- why?

(This is part of solving the original problem posed here: http://www.csharp-station.com/ShowPost.aspx?PostID=3625 What I need now is a method that accepts a ref to an arbitrary value type instance, or some other approach to the original problem. Ideas would be welcome.)

% cat u.cs
using System;

struct A {
  public A(int x, int y) { d_x = x; d_y = y; }
  public override string ToString()
    { return String.Format("({0},{1})", d_x, d_y); }
  //
  int d_x, d_y;
}

class RefTest {

  static void f(object obj)
    { Console.WriteLine("f: obj={0}", obj); }

  static void g(ref A obj)
    { Console.WriteLine("g: obj={0}", obj); }

  static void h(ref object obj)
    { Console.WriteLine("h: obj={0}", obj); }

  static void i(ref System.ValueType obj)
    { Console.WriteLine("i: obj={0}", obj); }

  static void Main()
  {
    A a = new A(3,2);

    f(a);

    g(ref a);

    // h(ref a);
    // u.cs(33,5): error CS1502: The best overloaded method match for 'RefTest.h(ref object)' has some invalid arguments
    // u.cs(33,11): error CS1503: Argument '1': cannot convert from 'ref A' to 'ref object'

    // i(ref a);
    // u.cs(37,5): error CS1502: The best overloaded method match for 'RefTest.i(ref System.ValueType)' has some invalid arguments
    // u.cs(37,11): error CS1503: Argument '1': cannot convert from 'ref A' to 'ref System.ValueType'
  }
}

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