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]

@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.

[ssegger1]

I am running some code analysis against a syntax tree containing the new field keyword using the Roslyn 4.12.0 libraries (currently the latest available). While walking the tree, the field keyword comes back as type IdentifierNameSyntax instead of the new type FieldExpressionSyntax. Using the Syntax Visualizer in VS 2022 v17.12.4, the node shows correctly as FieldExpressionSyntax.

[333fred]

You'd need to show a more complete example. At a guess, the parse options of that tree are not set to LanguageVersion.Preview, but that's just a guess.

[ssegger1]

The same thing occurs when I build and debug with the preview version of VS 2022 v17.13.0 Preview 5. I think an update to the Roslyn libraries is required, v4.12.0 was published on Dec 4, 2024, but it is the latest available.

[CyrusNajmabadi]

@ssegger1 You'd need to show a more complete example. Like a link to a repro project where you're experiencing the issue. As Fred said, it could be an issue like not setting the correct version, but we would not be able to tell without a proper repro.

[ssegger1]

Unfortunately I am not allowed to share company code.

[HaloFour]

A repro would be independent of company code. You should be able to reproduce the behavior in a completely independent and isolated project, which you can share.

[PoseidonEnergy]

Will we be able to do volatile reads of the field keyword using Volatile.Read(ref field) or something like that?

[CyrusNajmabadi]

Yes, you will.

[PoseidonEnergy]

Nice. I just tried the following and it seems to work:

public bool MyProperty => Volatile.Read(ref field);

Wonderful. This changes everything! No more manually creating volatile backing fields for each property!

[CyrusNajmabadi]

Glad you like it!

[jinsediaoying]

For a typical lazy create scene:

public MyObj Obj => field ??= new MyObj();

field will throw a nullcheck warning, because field is a not nullable type and require a initial value.
So I have to do:

[field: AllowNull]
public MyObj Obj => field ??= new MyObj();

Is there there any better way?

[PoseidonEnergy]

This works:

public MyObj Obj { get => field ??= new MyObj(); } = default!;

[jnm2]

This is an intermediate state of the feature (#8425) and the full nullability analysis implementation work is in progress: dotnet/roslyn#77127

The hope is for you not to have to use either workaround by the time the feature is released!

[jinsediaoying]

Oh thanks, then I will just wait.

[nikonthethird]

I wrote the following property in VS Code using .NET 10 Preview 3 with <LangVersion>preview</LangVersion>.

public sealed class State {
    private Int64 length;
    public ref Int64 Length => ref length;
}

I got a suggestion to convert this into an auto-property by the tooling. When I clicked on it, the code was converted to this:

public sealed class State {
    public ref Int64 Length => ref field;
}

This however displays error CS8145: Auto-implemented properties cannot return by reference.
Is this supposed to work or did the tooling suggest a conversion that does not work?

[franchyd]

Looks like a tooling issue

[jnm2]

Can you file this as a bug at https://github.com/dotnet/roslyn/issues?

[nikonthethird]

Can you file this as a bug at https://github.com/dotnet/roslyn/issues?

Sure, will do

Edit: Link to issue: dotnet/roslyn#78225

[znakeeye]

TL;DR   We need a PropertyMarshal.GetFieldRef() method.

I love this feature. Unfortunately, its use for view model properties is a bit limited. In our codebase, I could not use this feature for 70% of our properties due the limitation described below.

The docs state:
We believe the field keyword will find a lot of its use with view models because of the elegant solution it brings for the INotifyPropertyChanged pattern.

However, it is super common for view model properties to have some code when the value changes (see Prism/BindableBase which was originally developed by Microsoft). E.g.:

private object selectedItem;
public object SelectedItem
{
    get => selectedItem;
    set => SetProperty(ref selectedItem, value, OnSelectedItemChanged);
}

protected virtual void OnSelectedItemChanged()
{
}

Now, you can still convert this property to its elegant "field" equivalent. But you will face a problem!

The problem

In the constructor, you want to initialize the backing field. Not the property! For the property above, this is crucial, since you could otherwise introduce a performance hit and/or side effects in the onChanged action.

public MyViewModel(Guid id, object selectedItem)
{
     SelectedItem = selectedItem; // Uses property setter. No way to set the field directly when we have a setter!
     //
     SelectedItem = ResolveSelectedValue(id);
}

In all MVVM projects I have worked in, we have followed a strict rule to always set the backing field in the constructors. This has proven to be less error-prone than using the property setters.

Suggested improvement

In the constructor, allow setting the field directly. Something like this:

public MyViewModel(Guid id, object selectedItem)
{
     // Sets the field directly
     SelectedItem.init(selectedItem);
     
     // This could actually be useful outside the constructor!
     SelectedItem.field = selectedItem;
     
     // Just being creative ...
     fieldof(SelectedItem) = selectedItem;
     
     // Could be useful in other areas as well. Probably my favorite.
     ref SelectedItem = selectedItem;

     // Worst case, give us a helper method! Though I'm not sure how the method would pick up the property here...
     PropertyMarshal.GetFieldRef(SelectedItem) = selectedItem;
}

[CyrusNajmabadi]

TL;DR We need a PropertyMarshal.GetFieldRef() method.

I would open a separate discussion on this. Note, @RikkiGibson may already have one, with a hypothetical fieldof feature we could do at some point in the future if there's enough need.

[RikkiGibson]

#9031

[znakeeye]

Would be nice if the compiler could detect that the backing field should be Nullable<T> when leveraging the null-coalescing operator for value types:

internal class Foo
{
    [field: ThreadStatic]
    private static Guid ThreadGuid => field ??= new(); // CS0019
}

error CS0019: Operator '??=' cannot be applied to operands of type 'Guid' and 'new()'

[TwentyFourMinutes]

I would also wish this was possible, encounterd this multiple times.

[jnm2]

I would see this being addressed by #133:

private static Guid ThreadGuid { Guid? field; get => field ??= new(); }