Constructor vs Object Initializer Precedence in C#

问题

I've been learning the object initializer in C# recently, but now I'm wondering how it works when it conflicts with the constructor.

public class A
{
    public bool foo { get; set; }
    public A()
    {
        foo = true;
    }
    public A(bool bar)
    {
        foo = bar;
    }
}

What happens when I try this?

public class B
{
    a = A() {foo = false};
    b = A(true) {foo = false};
}

Is a default in the constructor a good way to have a bool that starts true and can be changed?

public A(bar=true)
{
    foo = bar;
}

回答1:

From the documentation:

The compiler processes object initializers by first accessing the default instance constructor and then processing the member initializations.

This means that in the simplest case (named object initialization) it is basically shorthand (or syntactic sugar) for calling the default constructor and then calling the property setter(s). In the case of anonymous types this kind of initialization is actually required and not mere sugar.

For the 2nd part of your question: It's more of a matter of style but if you have a crucial property I would not create a constructor with a default value. Make the client code set the value explicitly. I'm also not sure why doing something like this: b = A(true) {foo = false}; would be a good idea unless you're in a code obfuscation contest.

Bit of caution though:

... if the default constructor is declared as private in the class, object initializers that require public access will fail.

回答2:

Object initializers are just syntactic sugar, in your compiled assembly IL they translate into separate statements, check it on ILSpy.

回答3:

The constructor occurs first then the object initializer. Just remember that

a = new A() { foo = false };

is same as

var temp = new A();
temp.foo = false;
a = temp;

回答4:

b = new A(true) {foo = false};

is effectively short for:

A temp = new A(true);
temp.foo = false;
A b = temp;

where temp is an otherwise inaccessible variable. The constructor is always executed first, followed by any initialised properties.

回答5:

Essentially what Paul has already linked:

From the C# 5 language specification (7.6.10.1)

Processing of an object creation expression that includes an object initializer or
collection initializer consists of first processing the instance constructor and then
processing the member or element initializations specified by the object initializer or
collection initializer.

来源：https://stackoverflow.com/questions/17327266/constructor-vs-object-initializer-precedence-in-c-sharp

标签

constructor

object-initializers