Static constructor does not seem to be called?

Question

I have the following piece of code：

// using Windows.Storage;
internal static class AppData {
    private static ApplicationDataContainer container;
    public static bool FirstTime { get; } = !GetContainer("UserInfo").Values.ContainsKey("complete");
    static AppData() {
        container = ApplicationData.Current.LocalSettings;
    }
    private static ApplicationDataContainer GetContainer(string name) {
        return container.CreateContainer(name,ApplicationDataCreateDisposition.Always);
    }
}

NullReferenceException: Object reference not set to an instance of an object.

I don't know why it's wrong. Make some changes to the code

// using Windows.Storage;
internal static class AppData {
    private static ApplicationDataContainer container;
    public static bool FirstTime => !GetContainer("UserInfo").Values.ContainsKey("complete");
    static AppData() {
        container = ApplicationData.Current.LocalSettings;
    }
    private static ApplicationDataContainer GetContainer(string name) {
        return container.CreateContainer(name,ApplicationDataCreateDisposition.Always);
    }
}

OK, No error.

Why?

Did you check if ApplicationData is null or ApplicationData.Current is null ? — Praneet Nadkar, Mar 19 '18 at 12:06
This is not really a null reference question. In the first code chunk you have a property initializer, in the second you have a function definition. — BlackICE, Mar 19 '18 at 12:10
The first version sets up a read-only property (with backing field) and assigns to the backing field at static construction time. The second sets up a property (with just a getter) _without a backing field_. The second will work because, by the time you call it, `container` won't be `null`. — mjwills, Mar 19 '18 at 12:10
Possible duplicate of [Why static fields initialization occurs before the static constructor?](https://stackoverflow.com/questions/8285168/why-static-fields-initialization-occurs-before-the-static-constructor) — mjwills, Mar 19 '18 at 12:26

H H · Accepted Answer · 2018-03-19T12:39:21.987

6

I'll have to look up the reference but the issue is that

public static bool FirstTime { get; } = ....;

is an initializer. And as an initializer it is executed just before the constructor.

When you change it to a function (or readonly lambda property), it is a normal member and it gets executed after the constructor.

edited Mar 19 '18 at 12:39

answered Mar 19 '18 at 12:10

H H

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Here is that reference to save you lookup time: https://stackoverflow.com/a/8285210/5311735 – Evk Mar 19 '18 at 12:16
1

I second @mjwills. "=>" will make a read-only Prop without backing field, won't it? – Fildor Mar 19 '18 at 12:16
1

@Fildor it's a read-only property indeed. But read-only property is basically one function (like `GetFirstTime`), so distinction doesn't matter for this quesion. Still it's property and will be for example returned by `Type.GetProperty("FirstTime")`. – Evk Mar 19 '18 at 12:23
2

But you're right about the property, it is `bool FirstIme => ...` and a method would look like `bool FirstIme() => ...`. Semantics are the same. – H H Mar 19 '18 at 12:30

Martin Zikmund · Answer 2 · 2018-03-19T12:57:00.513

There is a big difference in the two syntaxes:

public static bool Property { get; set; } = true;

This is a property initialization syntax and is evaluated before any constructor runs. The property can have a setter as well here.

public static bool Property => true;

This is an expression property, get-only and the right hand side is evaluated only when it is called. This also means that it is evaluated each time it is accessed, so it is not a good design choice to use a complex method call on the right-hand side for example.

Also if you want to have the expression syntax while ensuring the right-hand side will be evaluated only when actually needed and only once, you can combine this syntax with a private field:

private static bool? _property;

public static bool Property => _property ?? ( _property = CalculateProperty() );

This will first check if the private backing field is null and if yes, initialize it, while the assignment itself will then return the assigned value as well.

Static constructor does not seem to be called?

2 Answers2