Can properties inside an object initializer reference each other?

问题

Is it somehow possible for properties to reference each other during the creation of ~~a dynamic object~~ an anonymously-typed object (i.e. inside the object initializer)? My simplified example below needs to reuse the Age property without making a second heavy call to GetAgeFromSomewhere(). Of course it doesn't work. Any suggestion on how to accomplish this?

var profile = new {
  Age = GetAgeFromSomewhere(id),
  IsLegal = (Age>18)
};

Is something like this possible or not possible with ~~dynamic objects~~ anonymously-typed object initializers?

回答1:

Unfortunately it's not possible, even with explicitly typed objects. This is because of the way object initializers work. For example:

public class MyClass
{
    public int Age = 10;
    public bool IsLegal = Age > 18;
}

Yields this compiler error at "IsLegal":

Error 1 A field initializer cannot reference the non-static field, method, or property 'MyClass.Age' ...

Field initializer can't reference other non-static fields, and since anonymous types don't create static fields, you can't use the value of one field to initialize another. The only way around this, is to declare the variables outside the anonymous type and use them inside the initializer.

int age = GetAgeFromSomewhere(id);
var profile = new {
  Age = age,
  IsLegal = age > 18
};

回答2:

Don't complicate thing, keep it simple

//Create a variable
var age = GetAgeFromSomewhere(id);
var profile = new {
  Age = age,
  IsLegal = age>18
}

回答3:

What you want is not possible within object intializers. You cannot read properties of the object being initialized. (It does not matter, whether the type is anonymous or not.)

Instead, Create a class

public class Profile
{
    public Profile(int id)
    {
        Age = GetAgeFromSomewhere(id);
    }

    public int Age { get; private set; }
    public int IsLegal { get { return Age > 18; } }
}

Or getting the age the lazy way:

public class Profile
{
    private readonly int _id;

    public Profile(int id)
    {
        _id = id;
    }

    private int? _age;
    public int Age {
        get {
            if (_age == null) {
                _age = GetAgeFromSomewhere(_id);
            }
            return _age.Value;
        }
    }

    public int IsLegal { get { return Age > 18; } }
}

or using the Lazy<T> class (starting with Framework 4.0):

public class Profile
{
    public Profile(int id)
    {
       // C# captures the `id` in a closure.
        _lazyAge = new Lazy<int>(
            () => GetAgeFromSomewhere(id)
        );
    }

    private Lazy<int> _lazyAge;
    public int Age { get { return _lazyAge.Value; } }

    public int IsLegal { get { return Age > 18; } }
}

Call it like this

var profile = new Profile(id);

来源：https://stackoverflow.com/questions/29719050/can-properties-inside-an-object-initializer-reference-each-other

标签

properties

object-initializers