Proposal: field keyword in properties

[lachbaer]

Proposal: field keyword in properties

(Ported from dotnet/roslyn#8364)

Specification: https://github.com/dotnet/csharplang/blob/main/proposals/field-keyword.md

LDM history:

• https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-03-10.md#field-keyword
• https://github.com/dotnet/csharplang/blob/main/meetings/2021/LDM-2021-04-14.md#field-keyword
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-01-12.md#open-questions-for-field
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-02-16.md#open-questions-in-field
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-03-02.md#open-questions-in-field
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-03-21.md#open-question-in-semi-auto-properties
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-05-02.md#field-questions
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-09-26.md#ungrouped
• https://github.com/dotnet/csharplang/blob/main/meetings/2022/LDM-2022-10-12.md#keywordness-of-field
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-05-15.md#field-and-value-as-contextual-keywords
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-06-24.md#field-questions
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-15.md#field-keyword
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-14.md#field-keyword
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-21.md#field-keyword-nullability
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-08-26.md#field-keyword-open-questions
• https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-09-18.md#nullability-in-field

[scottdorman]

This could also be useful/part of the implementation for #133.

[HaloFour]

My opinion is that this proposal makes auto-implemented properties more difficult to read by introducing these silent backing fields despite the use of logic. With expression-bodied accessor members a chunk of the boilerplate for standard properties already disappears. The IDE eliminates much of the rest via standard code snippets.

[lachbaer]

Suggestion:

• only allow setters to be defined in auto-properties (making them semi-auto)
• this setter expects the value to be assigned as a return value
• to set it in the middle of the block, yield return is used.

int ASemiAutoProp
{
    get;
    set
    {
        var pcea = new PropertyChangeEventArgs(this.ASemiAutoProp, value));
        OnPropertyChanging( pcea );
        yield return value;
        OnPropertyChanged( pcea );
    }
}

Then there is no need for a new field keyword/variable.

And the following really looks better than with #133

public T PropertyInitialized
{
    get;
    set => value ?? new ArgumentNullException();
} = default(T);

instead of (with #133) - a bit much of backingField.

public T PropertyInitialized
{
    T backingField = default(T);
    get => backingField;
    set => backingField = value ?? new ArgumentNullException();
}

[lachbaer]

Instead of using a field keyword, I also suggest following syntax borrowed from C struct:

public string FirstName
{
    get;
    set => _name = value;
} _name;

_name is by definition private to it's type (maybe unless directly prepended by an access modifier?).
OR just scoped to the corresponding property, what would even be better.

In case expression-bodied set of semi-auto-properties expect a value being returnd to assign to the backing-field (here with additional initializer):

public string FirstName
{
    get;
    set => value;
} _name = "(not set)";

This completely eradicates the need for a field keyword or yield return as suggested above, because now the backing field's identifier can be used

public string FirstName
{
    get;
    set {
        var pcea = new PropertyChangeEventArgs(_name, value));
        OnPropertyChanging( pcea );
        _name = value;
        OnPropertyChanged( pcea );
    }
} _name = "(not set)";

This would also go hand in hand with #133.

[HaloFour]

@lachbaer

With the declaration of the field being outside of the property block that would imply that the scope of that field would be for the entire class, not for just that property. That's also consistent with the scoping of that C syntax.

[lachbaer]

@HaloFour
Sure, but in this case we could probably rely on IntelliSense not to spoil the rest of the module.
Also without any prepending modifier it would imply internal access. (corrected, of course class members are private by default) 🙄
But nevertheless, I think that this is a neat way to achieve at least some of the goals.
Also the costs of implementing it could be reasonable.

[lachbaer]

Of course specifying a getter only works, too:

public string FirstName
{
    get {
        Debug.Assert( _name != null, "Getting name before setting it?");
        return _name;
    }
    set;
} _name = null;

or combined:

public string FirstName
{
    get {
        Debug.Assert( _name != null, "Getting name before setting it?");
        return _name;
    }
    set => value ?? throw new ArgumentNullException();
} _name;

In the latter example, just specifying the _name identifier qualifies for a semi-auto-property and therefore for set => value .

[jnm2]

@HaloFour

My opinion is that this proposal makes auto-implemented properties more difficult to read by introducing these silent backing fields despite the use of logic.

I don't disagree with your whole comment, but I wanted to point out that the difficulty reading is only if you're accustomed to thinking of logic and automatic backing fields as mutually exclusive. I don't think they should be. It's not hard to learn to look for a field keyword. It'll be right where you'll look if you're looking for the backing field anyway.

The reason I really like this proposal is not because it enables me to write less code, but because it allows me to scope a field to a property. A top source of bugs in the past has been inadequate control over direct field access and enforcing consistent property access. Refactoring into multiple types to add that desirable scope is quite often not worth it.

To that end, a field keyword makes perfect sense. I should not care or have to care what the backing field's name is. Ideally, it doesn't have one. This adds the convenience that renaming properties gets a lot easier.

Not infrequently I'm renaming constructs such as this, where Set handles INotifyPropertyChanged.PropertyChanged:

private bool someProperty;
public bool SomeProperty { get { return someProperty; } private set { Set(ref someProperty, value); } }

This kills two birds with one stone: 1) scope safety, 2) more DRY, less maintenence:

public bool SomeProperty { get; private set { Set(ref field, value); } }

[gafter]

/cc @CyrusNajmabadi

[lachbaer]

Meanwhile some time went by. I'd like to state my opionion on the questions from the inital post.

Allow both accessors to be defined simultaneously?

Yes, definitely. A nice example is the sample at the end of this comment. A semi-auto-property with an implicit field should be constructed under either or both these circumstances:

• both, getter and setter are declared, but either is get; or set; the other having a statement/block
• an initializer is declared and the property is not an auto-property

Assing expression bodied setters? and Assign block body with return?

I'd like to have that, because simply it looks nice and would totally fit into how "assign-to"-return expressions look.

But introducing to much new behaviour and especially having the compiler and reading user to differentiate between return and non-returning bodies can be confusing. Therefore I'd go with "no" on this currently.

Prohibit field keyword if not semi-auto?

No, but it must not be highlighted by the IDE in that case, because it that context it is no keyword anymore. I think it is very unlikely that somebody converts a semi-auto-property to an ordinary property and simultaneously has a 'field' named field in scope.

If property-scoped fields become availble shall that feature be available for semi-auto-properties as well?

Yes, if any possible. SAPs allow both, getter and setter, to be defined. It would make sense to make no difference versus normal properties to restrict that feature.

[jamesqo]

Taken from @quinmars at #681 (comment), this feature would allow devs to write a 1-liner to implement lazy initialization:

public T Foo => LazyInitializer.EnsureInitialized(ref field, InitializeFoo);

private T InitializeFoo() { ... }

[mattwar]

@jamesqo Except using this LazyInitializer method has a downside (unless the initializer method is static) because you'll be creating a delegate instance each time the property is accessed.

What you want to write is:

public T Foo => 
{
   if (field == null)
   {
        System.Threading.Interlocked.CompareExchange(ref field, InitializeFoo(), null);
   }

   return field;
};

And now its not really a one-liner.

[jamesqo]

@mattwar Well they said they intend to cache point-free delegates eventually. Also what if people don't care about extra allocations because the initializer is doing something like network I/O which completely dwarfs allocating a few extra objects?

[lachbaer]

public T Foo => LazyInitializer.EnsureInitialized(ref field, InitializeFoo);

However the field keyword should not be available to every property per se. It then should be decorated with a modifier, like e.g. public field T Foo ....

[yaakov-h]

@lachbaer why?

[CyrusNajmabadi]

I don't expect people to declare a local variable but not use it

You can declare it. We don't require @ in the declaration as there's no ambiguity as to what it means. Similar to the int from case above. It's completely unambiguous as to what it means. However, at the consumption point, you need to use @ as we need a way to know if you're referencing a variable or starting a new query construct.

The same is true for field. At the consumption point, we need to know if this is the special built-in auto-property backing field reference. Or if it's referencing one of our own variables named "field".

[jnm2]

I don't see how it is more of a breaking change to also require the declaration to use the '@' prefix. (I don't expect people to declare a local variable but not use it.)

The data showed it was tremendously more breaking. See https://github.com/dotnet/csharplang/blob/main/meetings/2024/LDM-2024-07-15.md#field-keyword for the meeting decision on this.

[jnm2]

@UniqeId6542 That's what we tried first, but GitHub data showed it broke too much code, so we had to dial it down.

[colejohnson66]

A bit of a tangent, but has the team considered creating auto-fixers like cargo fix for Rust?

[DanFTRX]

@UniqeId6542 That's what we tried first, but GitHub data showed it broke too much code, so we had to dial it down.

I believe you, but I am curious what cases are people declaring a local variable "field" in a property body then not accessing it? Or were people already using a @ in uses of the field but not the declaration?

[Boomkop3]

Perhaps I'm not understanding the idea wrong, but it looks like there's more than just a backing field being synthesized.

[CyrusNajmabadi]

@Boomkop3 what else do you see synthesized there? All i see is a synthesized backing List<string> field created there.

[Boomkop3]

@CyrusNajmabadi I am seeing =null!; but I didn't write set; or set => field = value;.

[CleanCodeX]

the property initalizer " = null!" is not part of the setter, it will be set as the initial value of the backing field.

[CyrusNajmabadi]

@CyrusNajmabadi I am seeing =null!; but I didn't write set; or set => field = value;.

Yes. That assigns to the backing field (just like a normal auto prop today). For example:

That didn't change with field.

[Boomkop3]

@CyrusNajmabadi ah, you're right, that's just for initialization. It's quite obvious now that I look at it again. Thank you!

[JimF42]

Have you considered giving the class access to the fields of other properties? I have on occasion needed to have one property's set accessor update a different property's backing field without accessing the set accessor.

[jnm2]

One of the motivations of the proposal was scoping, preventing this scenario. It's currently a reason to fall back to manually declaring a field.

[colejohnson66]

[zhuangState]

Does' field 'not support text interpolation?

[WeihanLi]

should be fixed in the latest VS preview version, see
#8634 (comment)
dotnet/roslyn#75566

[KieranDevvs]

Is this language version currently planned to be released as GA along side .NET 10 or will we potentially see it earlier?

[jnm2]

I don't believe there are any changes in the works for how C#'s releases line up with .NET's releases. That means C# 14 will be released at the same time as .NET 10.