C#•3w ago

✅ Backing Field and Primary Constructors

Hello guys, I was reading a bit about backing fields and primary constructors. Can someone explain when is a backing field created and when does a primary constructor argument persist please. For example, consider the following code:

C#
    private class Node(T t)
    {
        // No capture in this form. Data has a backing field, and you assign t to it.
        public T Data { get; set; } = t;
    }

    private class Node(T t)
    {
        // Capture in this form. Data has no backing field, and t is captured as class state
        public T Data => t;
    }

C#
    private class Node(T t)
    {
        // No capture in this form. Data has a backing field, and you assign t to it.
        public T Data { get; set; } = t;
    }

    private class Node(T t)
    {
        // Capture in this form. Data has no backing field, and t is captured as class state
        public T Data => t;
    }

For the first node class, I understood that t is not captured because we are assigning it directly to Data and we won't really use t later on and so it can be discarded. Data property also has a backing field. On the other hand, in the second node class, we don't have a backing field but t is captured. I don't understand why t is captured here and why is a backing field not generated, when is a backing field generated and when is the argument of a primary constructor captured please (just to clarify, a backing field is just a field that stores data, right?). If we don't have a backing field, this mean, we don't have a private field for something? It's as if the property doesn't exist ?

14 Replies

Pobiega•3w ago

What exactly do you think "capture" means/does here?

FakerOP•3w ago

hmm it creates a backing field for t ?

Pobiega•3w ago

yes in the second case

FakerOP•3w ago

hmm in second case, we have a backing field only for t, not Data? but hmm when we say "backing field", what do we mean by that, I mean, Data exist, go we can do something like Foo.Data Can we say the following (please can someone confirm): Backing field just means that a variable stores its own data. Capture means a backing field is created. So consider this:

C#
 private class Node(T t)
    {
        // No capture in this form. Data has a backing field, and you assign t to it.
        public T Data { get; set; } = t;
    }

C#
 private class Node(T t)
    {
        // No capture in this form. Data has a backing field, and you assign t to it.
        public T Data { get; set; } = t;
    }

Data has a setter, it stores its own data and so should have a backing field. t is not captured because we only assigned it to something; it's only used once. Now this:

C#
    private class Node(T t)
    {
        // Capture in this form. Data has no backing field, and t is captured as class state
        public T Data => t;
    }

C#
    private class Node(T t)
    {
        // Capture in this form. Data has no backing field, and t is captured as class state
        public T Data => t;
    }

Data only have a getter. Data depends on the value of t and doesn't store its own data. So Data doesn't have a backing field. Value of t is captured though because each time, it should be present so that using something like Foo.Data can retrieve the value?

Pobiega•3w ago

yes, because Data is just a getter, no setter. it has no state, its just a method

Backing field just means that a variable stores its own data.

a backing field is only a thing for properties that have state

FakerOP•3w ago

yeahh I see, backing field are used for properties which have a state?

Pobiega•3w ago

specifically for primary constructors, capture means a field is created yes

FakerOP•3w ago

yep I see, it's clearer now when we think of it in form of states hmm I think it's clearer now, will come back if I have other doubts, thanks !

Pobiega•3w ago

use sharplabs btw

FakerOP•3w ago

yeah just used it, it helped me a lot about backing field and all, just started to love using it 😂

Pobiega•3w ago

https://sharplab.io/#v2:CYLg1APgAgTAjAWAFBQMwAJboHIHtgCmAPACoB8AFCegC4CUyA3suq5htQCICGN36jdAHMCNANzoAvugC8tMcknJkaTDBz4CMUpWr0mLNqq69+MsvMVA here you can see both cases

FakerOP•3w ago

yep, thanks !!

333fred•3w ago

Capture means a backing field is created

Technically speaking, no. All you can say for certain is that it's captured The C# specification makes no guarantees on how a capture is actually stored I might not normally be pedantic like this, but if you're asking about the deep details on what the difference between these concepts are, then you get the pedantry

FakerOP•3w ago

yep I see

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✅ Backing Field and Primary Constructors

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