Field-backed properties: definite assignment #75116

cston · 2024-09-13T22:01:40Z

Similar to changes started in earlier features/semi-auto-props branch in #58832, #60601.

src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs

RikkiGibson · 2024-09-19T20:46:40Z

src/Compilers/CSharp/Portable/Binder/Binder.ValueChecks.cs

+                                        propertyAccess.ReceiverOpt,
+                                        propertyAccess.InitialBindingReceiverIsSubjectToCloning,
+                                        propertyAccess.PropertySymbol,
+                                        useAsLvalue.Value ? AccessorKind.Set : AccessorKind.Get, // PROTOTYPE: What about compound assignment, increment?


Consider extracting and reusing logic from 'GetIndexerAccessorKind' in order to determine this. #Resolved

RikkiGibson

Per offline discussion I am wondering if a somewhat smaller change would be adequate to address the scenario. It looks like the existing behavior with auto properties in struct constructors is:

property get: auto-default all unassigned fields, even if getter is auto-implemented.
property set: don't auto-default anything, if the set accessor is auto-implemented or absent.

So, shouldn't a relatively small change to logic in definite assignment be adequate here (check if setter specifically is auto-implemented rather than whole property)? There would be no need to adjust handling of property reads, compound assignments or increment/decrement.

cston · 2024-09-20T15:23:19Z

... It looks like the existing behavior with auto properties in struct constructors is:

property get: auto-default all unassigned fields, even if getter is auto-implemented.

property set: don't auto-default anything, if the set accessor is auto-implemented or absent.

Calling get with an auto property today will cause that one backing field to be initialized before the call but does not affect other fields that have yet to be assigned.

For instance, with the following, y = P1 results in initialization of P1 but not F1. See sharplab.io.

struct S1
{
    public int F1;
    public int P1 { get; }
    public S1(int x, out int y)
    {
        y = P1;
        F1 = x;
    }
}

src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs

RikkiGibson

Sorry, I still need more time to finish review, but had a few more comments.

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

RikkiGibson · 2024-09-20T21:38:19Z

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -1073,6 +1074,7 @@ static IEnumerable<Symbol> getAllMembersToBeDefaulted(Symbol requiredMember)
                            }
                        }

+                        // PROTOTYPE: Is checking IsAutoProperty sufficient or should we check the appropriate accessors are missing or auto-implemented?


I will try and resolve this comment in my upcoming PR. I think we should just check for a BackingField and return it here.

Moved comment to test plan #57012.

RikkiGibson · 2024-09-20T21:43:30Z

src/Compilers/CSharp/Portable/FlowAnalysis/NullableWalker.cs

@@ -1964,6 +1966,8 @@ protected override bool TryGetReceiverAndMember(BoundExpression expr, out BoundE
                        var propAccess = (BoundPropertyAccess)expr;
                        var propSymbol = propAccess.PropertySymbol;
                        member = propSymbol;
+                        // PROTOTYPE: Why don't we call Binder.AccessingAutoPropertyFromConstructor


I will also try to resolve this comment. I think I know why--nullable prefers to rewrite field initializers to be assigning to the property, for example, and then when a field is visited for constructor checks, we work back to the property and use the property's slot. Perhaps definite assignment works the opposite way, deciding when a property access is "morally" an access of the backing field, and treating it as the latter instead.

Moved comment to test plan #57012.

RikkiGibson · 2024-09-23T00:30:19Z

src/Compilers/CSharp/Portable/Compiler/MethodCompiler.cs

+            {
+                if (_found is { })
+                {
+                    return node;


Is this return value used? #Resolved

No, the return value is not used, and in fact most of the visit methods return null. I'll change this to return null;.

RikkiGibson

LGTM, nothing blocking.

RikkiGibson · 2024-09-23T18:23:32Z

src/Compilers/CSharp/Test/Emit3/FieldKeywordTests.cs

+                {
+                    public int F1;
+                    public int P1 { get; }
+                    public C1(int i) { P1 += i; }


It seems like this and the other scenarios in this test would be more illustrative if F1 were explicitly assigned after the P1 += i; statement. That would show whether the property access is causing auto-default of the whole instance.

Same for the ++, --, ??= tests below. #Resolved

Thanks, that makes sense.

RikkiGibson · 2024-09-23T18:58:01Z

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -2194,7 +2194,7 @@
    <!-- Whether receiver will need to be cloned during emit (only valid before lowering) -->
    <Field Name="InitialBindingReceiverIsSubjectToCloning" Type="ThreeState"/>
    <Field Name="PropertySymbol" Type="PropertySymbol"/>
-    <!-- The property accessor referenced if the property access should use the backing field directly instead. -->
+    <!-- The property accessor kind, set for properties with synthesized backing field only. -->
    <Field Name="UseBackingField" Type="AccessorKind" />


I'm not 100% happy with this field name, but I don't have any suggestions for a better one. It feels like this name is saying "this property access should be replaced with a use of the backing field". #Resolved

Renamed to BoundPropertyAccess.AutoPropertyAccessorKind.

RikkiGibson · 2024-09-23T20:14:05Z

src/Compilers/CSharp/Portable/BoundTree/BoundNodes.xml

@@ -2195,7 +2195,7 @@
    <Field Name="InitialBindingReceiverIsSubjectToCloning" Type="ThreeState"/>
    <Field Name="PropertySymbol" Type="PropertySymbol"/>
    <!-- The property accessor kind, set for properties with synthesized backing field only. -->


is it also the case that this will only have a non-Unknown value when auto accessors are being used? #Resolved

The value will be set to something other than AccessorKind.Unknown for classic auto properties and for properties that use field. Properties that use field may include an auto-implemented accessor.

Field-backed properties: definite assignment

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dotnet-issue-labeler bot added Area-Compilers untriaged Issues and PRs which have not yet been triaged by a lead labels Sep 13, 2024

cston added 3 commits September 13, 2024 15:11

Remove parameter

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More definite assignment; initial nullability support

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More definite assignment

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cston force-pushed the field-more branch 2 times, most recently from a1ecdc9 to 45ab7f9 Compare September 18, 2024 05:37

build-analysis bot mentioned this pull request Sep 18, 2024

Roslyn analyzer throws error AD0001 NullReferenceException dotnet/dnceng#3305

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Differentiate l-value and r-value property use in ctor

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cston force-pushed the field-more branch from 46ed4f4 to eb352eb Compare September 18, 2024 16:12

Cleanup; more tests

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Add BoundPropertyAccess.UseBackingField

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cston added 2 commits September 19, 2024 11:38

Fix test

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Cleanup

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cston marked this pull request as ready for review September 19, 2024 20:11

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src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs Show resolved Hide resolved

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Use GetAccessorKind(); additional test cases

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src/Compilers/CSharp/Portable/Binder/Binder_Statements.cs Show resolved Hide resolved

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cston added 3 commits September 21, 2024 07:50

Address feedback

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Fix tests

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Add visitor to verify backing field access

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Return null

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Cache accessor kind, ignoring accessor implementation

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Update comment

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cston added 3 commits September 23, 2024 12:29

Rename to BoundPropertyAccess.AutoPropertyAccessorKind

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Update tests to include field assignment

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Remove PROTOTYPE comments

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cston mentioned this pull request Sep 23, 2024

Test plan for "field keyword in properties" #57012

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cston merged commit 5aceebc into dotnet:features/field-keyword Sep 23, 2024
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NullableWalker.Scan().getFieldSymbolToBeInitialized(): Is checking IsAutoProperty sufficient or should we check the appropriate accessors are missing or auto-implemented #75245

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Field-backed properties: definite assignment #75116

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Field-backed properties: definite assignment #75116

Field-backed properties: definite assignment #75116

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