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Define capture behavior when primary ctor bypassed #7354

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Define capture behavior when primary ctor bypassed #7354

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df8c867
Define capture behavior when primary ctor bypassed
idg10 Jul 19, 2023
1f66023
Make wording less negative
idg10 Jul 20, 2023
4087b74
Use simpler no-constructor initialization example
idg10 Jul 21, 2023
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2 changes: 2 additions & 0 deletions proposals/primary-constructors.md
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Expand Up @@ -160,6 +160,8 @@ If a primary constructor parameter is referenced from within an instance member,

Capturing is not allowed for parameters that have ref-like type, and capturing is not allowed for `ref`, `in` or `out` parameters. This is similar to a limitation for capturing in lambdas.

There are circumstances in which constructors do not run. (For example, creating a single-element array of some struct type will produce an instance of that type without running any of its constructors.) Section [§9.2.5](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/variables#925-value-parameters) of the C# spec states that constructor parameters do not come into existence until the constructor is invoked. If a primary constructor parameter is captured in such a scenario, this would mean that the member causing its capture would be using a variable that does not exist. To avoid this, we assert that in cases where an instance of a type with a primary constructor was created through a mechanism that bypasses that constructor, all captured parameters come into existence when the instance is created, and are all initialized with the default value for their type.
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@CyrusNajmabadi CyrusNajmabadi Jul 21, 2023 •
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There are circumstances in which constructors do not run. (For example, creating a single-element array of some struct type will produce an instance of that type without running any of its constructors.) Section [§9.2.5](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/variables#925-value-parameters) of the C# spec states that constructor parameters do not come into existence until the constructor is invoked. If a primary constructor parameter is captured in such a scenario, this would mean that the member causing its capture would be using a variable that does not exist. To avoid this, we assert that in cases where an instance of a type with a primary constructor was created through a mechanism that bypasses that constructor, all captured parameters come into existence when the instance is created, and are all initialized with the default value for their type.
There are circumstances in which struct constructors do not run. For example `default(StructType)` produces such an instance. In those cases, despite no constructor being invoked (Section [§9.2.5](https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/language-specification/variables#925-value-parameters), all captured parameters of that struct-type come into existence when the instance is created, and are all initialized with the default value for their respective types.

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i would also add:

For instance for this declaration and code:

``` c#
public struct Point(int x, int y)
{
    public override string ToString() => $"Point({x}, {y});
}

var point = default(Point);
var text = point.ToString();
```

"text" will have the value `"Point(0, 0)"` due to the captured `x` and `y` parameters having default values.


If a primary constructor parameter is only referenced from within instance member initializers, those can directly reference the parameter of the generated constructor, as they are executed as part of it.

Primary Constructor will do the following sequence of operations:
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