CLR question. Why method overloading in C# decides that null is a string? [duplicate]

Question

Possible Duplicate:
C#: Passing null to overloaded method - which method is called?

Here is a test case

object a = null;
var b = Convert.ToString (null);
var c = Convert.ToString (a);
string d = Convert.ToString (null); // CLR chooses Convert.ToString(string value)
string e = Convert.ToString (a); // CLR chooses Convert.ToString(object value)

The question is why CLR decides that null is interpreted as string in first case? It appears that this question was already answered here

Here is another similar case. None of these ifs are triggered

object x = null;
if (x is object)
{
    Console.Write ("x is object");
}

if (x is string)
{
    Console.Write ("x is string");
}

if (null is object)
{
    Console.Write ("null is object");
}

if (null is string)
{
    Console.Write ("null is string");
}

Answer 1

The answer is because it must choose a reference type (null doesn't work for value types), and every string is an object, but not every object is a string. See Jon Skeet's answer to this question for more information.

In response to your second example, if a variable that is null is passed to is, it will always evaluate to false, no matter what.