为什么这个嵌套对象初始化器抛出一个空引用异常？

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
                          },
                      };
    }
}

Answer 1

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, ie 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 .