C#•17h ago

? operator and property assigned to =null! meaning in EF core database context

C#

using System;
using System.Collections.Generic;

namespace Learning.Models;

public partial class Book
{
    public int BookId { get; set; }

    public string Title { get; set; } = null!;

    public string? Author { get; set; }

    public string? Isbn { get; set; }

    public int? PublishedYear { get; set; }
    
    public DateTime Date { get; set; }
    public string BookVersion { get; set; }

    public virtual ICollection<Rental> Rentals { get; set; } = new List<Rental>();

    public override string ToString()
    {
        return $"Title: {Title}, Author: {Author}, ISBN: {Isbn}";
    }
}

C#

using System;
using System.Collections.Generic;

namespace Learning.Models;

public partial class Book
{
    public int BookId { get; set; }

    public string Title { get; set; } = null!;

    public string? Author { get; set; }

    public string? Isbn { get; set; }

    public int? PublishedYear { get; set; }
    
    public DateTime Date { get; set; }
    public string BookVersion { get; set; }

    public virtual ICollection<Rental> Rentals { get; set; } = new List<Rental>();

    public override string ToString()
    {
        return $"Title: {Title}, Author: {Author}, ISBN: {Isbn}";
    }
}

Hello guys, this is a book class that I scaffolded using EF Core. I have a few questions. What does that mean when we use = null! and what does that mean when we say a type of associated with ?. What does it convey in terms of database context, like nullable/ not nullable? I know that ? is used to say that we allow a data type to have null value. But what does it mean here? Also, =null basically means we are assigning null as soon as we are creating, but what does the ! mean then? Whys is the =null! important please, can we omit it?

27 Replies

Sehra•17h ago

? is the same, property can be null, mapped on database side as nullable

Angius•17h ago

I know that ? is used to say that we allow a data type to have null value. But what does it mean here?

ERxactly that

FakerOP•17h ago

oh ok, without the ? operator, we would have the NOT NULL constraint on the column in the database?

Angius•17h ago

FakerOP•17h ago

ohhhh, ok, hmm what about the null! pls

Sehra•17h ago

null! is just to silence a compiler warning that a non-nullable property can be null after exiting the constructor. only doing =null would still be a warning, so null! use the null-forgiving operator to say that this null shouldn't trigger a warning

Angius•17h ago

! is a null-forgiving operator, meaning "trust me bro, this is not null" Generally, using required properties is a better idea

FakerOP•17h ago

yeah I see, but when we say a warning, is it something that we won't be able to run the code with? it's mandatory here?

Angius•17h ago

But EF can be wonky about it

Sehra•17h ago

since the database will not allow null in the column, when it comes back it will be set to a non-null value

Angius•17h ago

No, warning is just a warning

Jimmacle•17h ago

the problem with EF is that querying on entities with optional relationships doesn't match up with C#'s understanding of the type just because it's nullable in the C# type doesn't mean you need to check it for null in your EF LINQ queries so you have to do some level of bending NRTs to your will also, even if it's a required relationship it may still be null in the C# object if you didn't .Include the navigation

FakerOP•17h ago

hmm so this give us another question :c : When to use the null! thing? Like if I've understood correctly, we are saying ok, this thing cannot be null but we are still assigning null to it but ignore the warning, we will pass a value to it later on.

Angius•16h ago

Outside of ef models, never In ef models, on nav props

Sehra•16h ago

class SomeEntity {
  public int OtherEntityId {get;set;}
  public OtherEntity? OtherEntity {get;set;}
}

class SomeEntity {
  public int OtherEntityId {get;set;}
  public OtherEntity? OtherEntity {get;set;}
}

this is usually the pattern for navigation properties the id is not nullable, the full entity is. if you want it populated, include it it's mostly string messing it up since it's a reference type

Angius•16h ago

Outside of nav props, I'd definitely opt for either required, or a default value Could be an empty string or a zero even, but not null!

FakerOP•16h ago

I just remove the =null! in title and my IDE gives me this warning

FakerOP•16h ago

So the thing is, anything declared as non-nullable (we don't have the ? operator should be assigned to something at compile-time to remove the warning ?)

Sehra•16h ago

doesn't have to be set at compile time it warns if it is not set after constructor exits

FakerOP•16h ago

yep I see if we don't pass a value for it in the constructor, we would get an exception?

Sehra•16h ago

nope

Jimmacle•16h ago

NRTs are not a runtime feature it's just for the compiler to help you avoid writing bugs nothing actually stops you from setting a non-nullable variable to null

FakerOP•16h ago

yep I see Ok guys, just need to do small summary, please confirm whether the following is correct/add up to it if needed pls: 1. ? is used to explicitly say that we are expecting a particular column to be null in the database context. For example, int PublishedYear means this is a nullable field. 2. If we have reference types, like string, the default value of string and any other referenced type is null. But without the ?, we need to provide a value. We provide a value, like null then we just suppress the warning using the null-forigiving operator !. (Would have been better to use a default value here @ZZZZZZZZZZZZZZZZZZZZZZZZZ ? why pls like you said either use default value or required attribute; what does the required attribute do in this case?) 3. Also, just read about the Include() thing. Basically, the nav property which linked a particular object to another entity, that object can be null if it's not loaded; that's where it might be useful to use the ? operator. Now, in order for the object, not to be null, we use the Include() method which loads them "eagerly" something like that.

Angius•16h ago

1. int PublishedYear means it is not nullable 2. Basically, yeah. And not a [Required] attribute, but a required keyword 3. Yep. Keep in mind though, that it's usually a better idea to .Select() into some DTO class rather than having a bunch of includes. Fetch only what you need.

FakerOP•15h ago

yeah I see, in the DTO thing, we would have 2 queries then, like we wouldn't be able to use a single query to fetch both a subset of Book data and use that in our next query?

Angius•15h ago

A single query Well, a single query to get the book and whatever related data If you wanted to, say, create a copy of this entry, that would be a second query, yeah But using .Include() would not change that

FakerOP•15h ago

yep I see, thanks !

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? operator and property assigned to =null! meaning in EF core database context

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