enum method fired when object should be - c#-3.0

Question

I a sample C# console application to display a bug I am experiencing:
class Program
{
public enum Days { Sat = 1, Sun, Mon, Tue, Wed, Thu, Fri };
static void Main(string[] args)
{
AddWhere("a", DateTime.Now);
AddWhere("a", 0);
AddWhere("a", 2);
AddWhere("a", 3);
AddWhere("a", "4");
AddWhere("a", Days.Sun);
AddWhere("a", Days.Fri);
AddWhere("a", 1);
AddWhere("a", (int)Days.Sat);
Console.Read();
}
public static void AddWhere(string columnName, Days cd)
{
Console.WriteLine("enum fired");
}
public static void AddWhere(string columnName, object Val)
{
Console.WriteLine("object fired");
}
}
the output I get is this:
object fired
enum fired
object fired
object fired
object fired
enum fired
enum fired
object fired
object fired
Why does the enum method fire when 0 is passed in?

Answer 1

The special case of 0 is covered in section 1.10 of the C# language specification.
In order for the default value of an enum type to be easily available, the literal 0 implicitly converts to any enum type
This implicit conversion is causing overload resolution to pick the enum overload over the object one.

Answer 2

JaredPar answered the question. I will add that the work-around is to cast the 0 as the exact type of the desired method overload.
AddWhere("a", (object)0);

Answer 3

Because enums are stored by default as integers and when the compiler tries to resolve the best method overload it decides that AddWhere(string columnName, Days cd) is a better match.

Answer 4

Enum fired because 0 is an int and the enum underlying type is int. 0 is implicitly converted to enum (at compile time) as this conversion is defined by the language.
int a = 123;
long b = a; // implicit conversion from int to long
int c = (int) b; // explicit conversion from long to int
Some conversions are defined by the language
Source: msdn.microsoft.com