SA1129 : C# 10 Re-evaluation

[Foxtrek64]

Hello,

SA1129 as many of you know is a rule that has the developer prefer default(T) over new T() for value types. Historically, these were functionally equivalent and the former was preferred to avoid confusion and to make code consistent.

With C# 10, developers are allowed to create parameterless constructors for their structs, which means that these two forms are no longer (necessarily) equivalent.

As such, it is likely time to re-evaluate this rule and discuss whether to rescope or to retire this rule entirely.

I personally see three possibilities:

1. Rescope entirely to builtin value types (prefer default(int) over new int()).
2. Rescope entirely to builtin value types and invert preference (new int() or new() over default(int)).
3. Retire entirely (possibly implementing 1 or 2 separately as a new rule).

[sharwell]

I believe the current rule is still correct for almost all cases. The cases to consider, if any, would be:

1. The behavior for new T() where T is a generic type parameter constrained to struct
2. The behavior for new T() where T is a value type known to have a user-defined parameterless constructor in metadata

I believe the risk associated with unintentional creation of an invalid instance is sufficient that case (1) should retain the current behavior. Case (2) is a straightforward false positive that can be addressed at any time.

[sharwell]

I fixed this with the conservative approach.

[alexrp]

@sharwell What would be the recommendation for types like SpinWait? It doesn't have an explicit parameterless constructor and is designed to work default-initialized. I suspect most people would find var wait = new SpinWait(); ...; wait.SpinOnce(); more clear than var wait = default(SpinWait); ...; wait.SpinOnce();. But SA1129 wants the latter.

[sharwell]

I believe the simplest recommendation is to just stick to the main rule. This is what the runtime repository itself does with SpinWait:
https://github.com/dotnet/runtime/blob/e48558390c988a830040e355c55f729615c91246/src/libraries/System.Private.CoreLib/src/System/Threading/SpinWait.cs#L319