Question

If I compile the following code snippet with Visual C# 2010 I ALWAYS get false:

object o = null;
Console.WriteLine("Is null: " + o == null); // returns false

Does anybody know why???

Answer 1

Why is easy; think of what you’ve written as actually being this:

object o = null;
Console.WriteLine(("Is null: " + o) == null); // returns false

It’s testing "Is null: " + o against null, which will always be false. This is due to the rules of operator precedence, where + comes before ==.

You should explicitly apply parens to make sure it’s working like you want:

Console.WriteLine("Is null: " + (o == null)); // returns true

As noted in the comments by Jim Rhodes:

This is one of several reasons why you should ALWAYS use parentheses and never rely on compiler precedence rules.

I noted myself that I agree; that I don’t even try to remember operator precedence rules myself, instead being explicit with parens all the time. I further suggest that this is also one reason to be very careful when relying on implicit type conversion and/or methods with multiple overloads.

I’d also like to point out that I really like something Ravadre noted in their answer; about why only “False” was printed, and not the whole text you were trying to print.