Why does the following conditional operator "?:" compile but give a run-time error

Question

I cannot figure out why the following program compiles without warnings, but in the exception block the conditional operator gives a NullReferenceException.

using System;

namespace Playground
{
    class Program
    {
        static void Main(string[] args)
        {
            string message1 = "First message.";
            string message2 = null;
            // This works without a problem
            Console.WriteLine(message1 + message2 == null ? "" : Environment.NewLine + message2);
            Console.ReadKey();

            try
            {
                throw new Exception("Now with an exception.");
            }
            catch (Exception ex)
            {
                // This will give a NullReferenceException:
                Console.WriteLine(ex.Message + ex.InnerException == null ? "" : Environment.NewLine + ex.InnerException.Message);
                // ..But this will work:
                Console.WriteLine(ex.Message + (ex.InnerException == null ? "" : Environment.NewLine + ex.InnerException.Message));
            }
        }
    }
}

I know about the ?? operator, my question is about why the first line in the exception handler gives a NullReferenceException.

Answer 1

ex.Message + ex.InnerException

is converted into

string.Concat(ex.Message, ex.InnerException);

this checks its arguments for null and only converts them to strings if they are not. This means that

ex.Message + ex.InnerException == null

is false so the expression

Environment.NewLine + ex.InnerException.Message

is evaluated. This throws an exception since ex.InnerException is null.