compiler accepts almost-object-initializer that throws NullReferenceException [duplicate]

Question

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Closed 9 years ago.

Possible Duplicate:
Initializer syntax

Short code sample to demonstrate (VS2010 SP1, 64-bit Win7):

class A
{
    public string Name { get; set; }
}

class B
{
    public A a { get; set; }
}

// OK
A a = new A { Name = "foo" };
// Using collection initialiser syntax fails as expected:
// "Can only use array initializer expressions to assign
// to array types. Try using a new expression instead."
A a = { Name = "foo" };

// OK
B b = new B { a = new A { Name = "foo" } };
// Compiles, but throws NullReferenceException when run
B b = new B { a = { Name = "foo" } };

I was surprised to see that last line compile and thought I'd found a nifty (though inconsistent) shortcut before seeing it blow up at runtime. Does that last usage have any utility?

Answer 1

The last line is translated to:

B tmp = new B();
tmp.a.Name = "foo";
B b = tmp;

And yes, it very definitely has utility - when the newly created object has a read-only property returning a mutable type. The most common use of something similar is probably for collections though:

Person person = new Person {
    Friends = {
        new Person("Dave"),
        new Person("Bob"),
    }
}

This will fetch the list of friends from Person and add two new people to it.