对象初始化期间的 NullReferenceException

Question

Why there's a NullReferenceException when trying to set value of X in the code below? It works fine when I use new keyword when initializing B , but why it compiles fine without new and then fails during runtime?

https://dotnetfiddle.net/YNvPog

public class A
{
    public _B B;
    public class _B
    {
        public int X;
    }
}

public class Program
{
    public static void Main()
    {
        var a=new A{
                B={
                    X=1
                }
            };
    }
}

Answer 1

Initialization syntax can be tricky. In your code, you're trying to set the value of aBX without first setting the value of B . Your code translates to:

var a = new A();
a.B.X = 1;

... which would produce the same exception you're getting now. That's because aB is initialized to null unless you explicitly create an instance for it.

As you noted, this will work:

    var a=new A{
            B= new _B {
                X=1
            }
        };

You could also make sure that A 's constructor initializes a B .

    public _B B = new A._B();

why it compiles fine without new and then fails during runtime?

It would require too much work for the compiler to dig into the code for your A class and realize that B will definitely be null at this point in time: as I pointed out you could change the implementation of A 's constructor to make sure that's not the case. This is one reason that null reference exceptions are the most common type of exception out there.

The best strategy to avoid this is to initialize all of your fields to non-null values in the constructor. If you won't know what value to give them until your constructor is invoked, then make your constructor take those values as parameters. If you expect one of your fields may not always have a value, you can use an optional type like my Maybe<> struct to force programmers to deal with that fact at compile-time.

Update 2021

Now C# supports nullable reference types , which you can use to encourage/force programmers to know that your field could be null, if that's the route you want to take.

    public _B? B;