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Field-backed properties: readonly backing field #74922

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Aug 29, 2024
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Field-backed properties: readonly backing field #74922

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da51e61
Field-backed properties: readonly backing field
cston Aug 27, 2024
0b915a6
Add test for readonly accessors
cston Aug 28, 2024
4a962a9
Clarify comment
cston Aug 29, 2024
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37 changes: 28 additions & 9 deletions src/Compilers/CSharp/Portable/Symbols/Source/SourcePropertySymbolBase.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -172,15 +172,34 @@ protected SourcePropertySymbolBase(
{
Debug.Assert(!IsIndexer);
string fieldName = GeneratedNames.MakeBackingFieldName(_name);
BackingField = new SynthesizedBackingFieldSymbol(this,
fieldName,
// Synthesized backing field for 'field' should not be marked 'initonly'
// since the field might be modified in the get accessor.
// PROTOTYPE: Should the backing field be 'initonly' when the containing
// type, property, or accessor is declared 'readonly'?
isReadOnly: !usesFieldKeyword && ((hasGetAccessor && !hasSetAccessor) || isInitOnly),
this.IsStatic,
hasInitializer);

// The backing field is readonly if any of the following holds:
// - The containing type is declared readonly and the property is an instance property.
bool isReadOnly;
if (!IsStatic && containingType.IsReadOnly)
{
isReadOnly = true;
}
// - The property is declared readonly.
else if (HasReadOnlyModifier)
{
isReadOnly = true;
}
// - The property has no set accessor (but may have an init accessor) and
// the get accessor, if any, is automatically implemented.
else if ((!hasSetAccessor || isInitOnly) && (!hasGetAccessor || hasAutoPropertyGet))
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Should the field also be readonly if all manually implemented get and set accessors are declared readonly?

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I have a slight leaning towards "no", but not a strong opinion.

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It's not valid to declare both accessors as readonly, you'd have to declare the containing property as readonly instead, which would make the field readonly per these rules.

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Declaring both as readonly is an error, but we may want to treat the backing field as readonly in the following case:

struct S
{
    object P { get; readonly set { } }
}

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I think a more compelling example is:

struct S
{
    object P { readonly get => field; init; }
}

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FWIW, I can't think of a case where things are made worse by the backing field not being readonly, in cases involving the accessors being marked readonly but not the containing type or property.

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I've added a PROTOTYPE comment and a test for now.

{
isReadOnly = true;
}
else
{
// PROTOTYPE: We could treat the field as readonly if all manually implemented get and set
// accessors are declared readonly. Although, to do so, we might need to bind the accessor
// declarations before creating the backing field. See FieldKeywordTests.ReadOnly_05().
isReadOnly = false;
}

BackingField = new SynthesizedBackingFieldSymbol(this, fieldName, isReadOnly: isReadOnly, this.IsStatic, hasInitializer);
}

if (hasGetAccessor)
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