myString = myObject.myValue 
         [?? or some other GuardOperator Here] "Default Value";

… is straight forward enough. But what if someone were to write this:

myString = myObject.mySubObject.myValue 
           [GuardOperator Here] "Default Value";

Now should the compiler set the default value only if mySubObject is null? (and throw an exception if myObject is null). Or should it guard against the entire expression altogether?

Because it isn’t really useful if doesn’t guard against the entire expression, I guess what you’re really asking for is a shortcut for catching NullReferenceExcptions as opposed to a shortcut for an if-else clause. Something like this:

string myString;
try
{
	myString = myObject.mySubObject1.mySubObject2.Value;
}
catch(NullReferenceException e)
{
	myString = "Default Value";
}

… and I agree that would indeed be useful. (The Javascript guard operator is a little odd because it can return either myValue or myObject.)
Its a pity one can’t really write a method for doing this. (In C# that is) You can use reflection but then you’d lose the strong typing.

string myString = myObject.Value ?? “default value”;

as in, I want whatever Value is, as long as myObject is not null, otherwise the default value. Which obviously ?? does not accomplish, but I just find this scenario comes up more often.

have to stick with the old standby…
string myString = myObject == null ? “default value” : myObject.Value;