Why does this nested object initializer throw a null reference exception?

Question

The following testcase throws a null-reference exception, when it tries to assign Id to an object which is null, since the code is missing the "new R" before the object initializer.

Why is this not caught by the compiler? Why is it allowed, in which use-cases would this be a meaningful construct?

[TestClass]
public class ThrowAway
{
    public class H
    {
        public int Id { get; set; }
    }

    public class R
    {
        public H Header { get; set; }
    }

    [TestMethod]
    public void ThrowsException()
    {
        var request = new R
                      {
                          Header =
                          {
                              Id = 1
                          },
                      };
    }
}

You have to initialize the Header property of your R object. var request = new R { Header = new H { Id = 1 } }; — Styxxy
– Styxxy, Commented Jun 3, 2015 at 13:00
Very interesting question... it isn't even clear if the compiler is doing a cast or what... — xanatos
– xanatos, Commented Jun 3, 2015 at 13:04
Very interesting question, but extremely poor title in my opinion. Would you consider changing it to the actual question you are asking? — julealgon
– julealgon, Commented Jun 3, 2015 at 13:13
That's a lot better I think (you missed the question mark at the end :P ). Upvoted now. — julealgon
– julealgon, Commented Jun 3, 2015 at 13:52

xanatos · Accepted Answer · 2015-06-03 13:15:32Z

19

The compiler doesn't give a warning because you could have:

public class R
{
    public H Header { get; set; }

    public R()
    {
        Header = new H();
    }
}

so Header could be initialized by someone/something. Solving if someone/something will initialize Header is a complex problem (probably similar to the Halting problem)... Not something that a compiler wants to solve for you :-)

From the C# specifications:

A member initializer that specifies an object initializer after the equals sign is a nested object initializer, i.e. an initialization of an embedded object. Instead of assigning a new value to the field or property, the assignments in the nested object initializer are treated as assignments to members of the field or property. Nested object initializers cannot be applied to properties with a value type, or to read-only fields with a value type.

We are in the case of nested initializer, and see the bolded part. I didn't know it.

Now, note that new R { } is, by C# spec, an 7.6.10.1 Object creation expressions followed by an object-initializer, while the Header = { } is a "pure" 7.6.10.2 Object initializers.

edited Jun 3, 2015 at 13:15

answered Jun 3, 2015 at 13:06

xanatos

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5 Comments

Andrei Over a year ago

Same question as to the another post here - why did it allow Header = {Id = 1}? This looks plain wrong anyway, doesn't it?

xanatos Over a year ago

@Andrei At least I explained the Why is this not caught by the compiler? :-)

Andrei Over a year ago

Sorry, I still don't get it. Not trying to argue, just very curious myself. Compiler should have failed on such syntax, no matter if there is initialization in ctor, right?

xanatos Over a year ago

@Andrei Found the part of C# spec that defines this case. My response was more in the tone of "why isn't there a warning" (and in fact my response starts with a The compiler doesn't give a warning because you could have:)

Hugo Hallqvist Over a year ago

Thanks xanatos, I think I can see why it can't warn or fail now. Thanks!

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