Visual Studio 2015 missing new keyword - no compiler error

Question

I was working on one of our new applications when I made a stupid mistake… I forgot to write: ”new ClassName” inside an object initializer. Strangely VS just compiled the code… without any error or warning. (I’ve used VS 2015, .Net 4.5.2)

My Code:

var target = new MyClass
{
    MyValues = new List<MyOtherClass>
    {
        new MyOtherClass
        {
            Start = new DateTimeValue
            {
                DateTime = new DateTime(2015, 08, 23)
            },
            End = {
                DateTime = new DateTime(2015, 08, 23)
            }
        }
    }
};

(Start and End are both of type DateTimeValue)

When I start the application, this code has thrown a NullReferenceException. (Adding “new DateTimeValue” fixed the problem). Why was there no error from the compiler?

As an aside, this would be a better question with a short but *complete* example - we don't need both `MyClass` and `MyOtherClass`, nor a `List` getting involved... but a short but complete example we could run would be great. — Jon Skeet, Aug 26 '15 at 15:00

score 9 · Accepted Answer · answered Aug 26 '15 at 14:52

There's nothing wrong there at all. It's using an object initializer for End which sets properties using the existing value instead of assigning a new value to End. Your code is equivalent to:

var tmp0 = new MyClass();
var tmp1 = new List<MyOtherClass>();
var tmp2 = new MyOtherClass();
var tmp3 = new DateTimeValue();
tmp3.DateTime = new DateTime(2015, 8, 23);
// Note the assignment to Start here
tmp2.Start = tmp3;

// No assignment to End -
// it's calling the End "getter" and then the DateTime "setter" on the result
tmp2.End.DateTime = new DateTime(2015, 8, 23);
tmp1.Add(tmp2);
tmp0.MyValues = tmp1;
var target = tmp0;

From the C# 5 specification, section 7.6.10.2:

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.

That's exactly what you're doing here - the { DateTime = ... } part is an object initializer.

Lucas Trzesniewski · Answer 2 · 2015-08-26T14:58:15.140

Because that's valid syntax.

It translates roughly to the following:

var target = new MyClass();
target.MyValues = new List<MyOtherClass>();

var i = new MyOtherClass();

i.Start = new DateTimeValue();
i.Start.DateTime = new DateTime(2015, 08, 23);

i.End.DateTime = new DateTime(2015, 08, 23);

target.MyValues.Add(i);

Except the properties are only set once, and are never read by the compiler. It would require using additional temporary variables (like Jon did in his answer), but I didn't bother writing them here for simplicity.

This syntax is uselful for the case when MyOtherClass instantiates the End property in its constructor. It can also be used when the property is read only.

This explains it… didn’t know that code like this works. Thank you very much. — musium, Aug 26 '15 at 14:52

Visual Studio 2015 missing new keyword - no compiler error

2 Answers2