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Nullability: conversion behavior #1242

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Nullability: conversion behavior #1242

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jnm2 Jan 1, 2025
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jnm2 Jan 1, 2025
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Nullability: conversion behavior
jnm2 Jan 1, 2025
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jnm2 Jan 1, 2025
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90 changes: 83 additions & 7 deletions standard/types.md
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Expand Up @@ -859,6 +859,8 @@

**The remainder of this subclause is conditionally normative.**

#### 8.9.5.1 Flow analysis

A compiler that generates diagnostic warnings conforms to these rules.

Every expression has one of three ***null state***s:
Expand Down Expand Up @@ -925,10 +927,10 @@
> int length = p.Length; // Warning: p is maybe null
>
> string s = p; // No warning. p is not null
>
>
> if (s != null)
> {
> int l2 = s.Length; // No warning. s is not null
> int l2 = s.Length; // No warning. s is not null
> }
> int l3 = s.Length; // Warning. s is maybe null
> }
Expand All @@ -946,13 +948,13 @@
> public void M(string s)
> {
> int length = s.Length; // No warning. s is not null
>

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MDC032::Line length 87 > maximum 81
> _ = s == null; // Null check by testing equality. The null state of s is maybe null
> length = s.Length; // Warning, and changes the null state of s to not null
>

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> _ = s?.Length; // The ?. is a null check and changes the null state of s to maybe null
> if (s.Length > 4) // Warning. Changes null state of s to not null
> {

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MDC032::Line length 87 > maximum 81
> _ = s?[4]; // ?[] is a null check and changes the null state of s to maybe null
> _ = s.Length; // Warning. s is maybe null
> }
Expand Down Expand Up @@ -998,13 +1000,13 @@
> {
> get
> {
> string tmp = _field;
> _field = null;
> return tmp;
> string tmp = _field;
> _field = null;
> return tmp;
> }
> set
> {
> _field = value;
> _field = value;
> }
> }
>
Expand All @@ -1012,7 +1014,7 @@
> {
> var t = new Test();
> if (t.DisappearingProperty != null)
> {

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> int len = t.DisappearingProperty.Length; // No warning. A compiler can assume property is stateful
> }
> }
Expand All @@ -1031,7 +1033,7 @@
> public class C
> {
> private C? child;
>
>
> public void M()
> {
> _ = child.child.child; // Warning. Dereference possible null value
Expand All @@ -1042,4 +1044,78 @@
>
> *end example*

#### 8.9.5.2 Type conversions

For the purpose of determining whether a conversion is *permitted*, a compiler must consider every nullable-annotated type to be equivalent to its unannotated version. The compiler may however issue warnings if the annotations of the types are not compatible.

(examples: `List<string>` to `IEnumerable<object?>`, or `List<string?>?` to `IEnumerable<object>`, ...)

A compiler may follow rules for interface variance ([§18.2.3.3](interfaces.md#18233-variance-conversion)), delegate variance ([§20.4](delegates.md#204-delegate-compatibility)), and array covariance ([§1.7.6](arrays.md#176-array-covariance)) in determining whether to issue a warning for type conversions.

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(Do we need to list each type here? E.g. tuple types...)

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MDC019::Mismatch: link anchor is '§1.7.6', should be '§17.6'

> <!-- Example: {template:"code-in-class-lib", name:"NullVariance"} -->
> ```csharp
> #nullable enable
> public class C
> {
> public void M1(IEnumerable<string> p)
> {
> IEnumerable<string?> v1 = p; // No warning
> }
>
> public void M2(IEnumerable<string?> p)
> {
> IEnumerable<string> v1 = p; // Warning
> IEnumerable<string> v2 = p!; // No warning
> }
>
> public void M3(Action<string?> p)
> {
> Action<string> v1 = p; // No warning
> }
>
> public void M4(Action<string> p)
> {
> Action<string?> v1 = p; // Warning
> Action<string?> v2 = p!; // No warning
> }
>
> public void M5(string[] p)
> {
> string?[] v1 = p; // No warning
> }
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@jnm2 jnm2 Jan 1, 2025 •
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Yes. https://github.com/dotnet/csharplang/blob/main/meetings/2017/LDM-2017-10-04.md#array-covariance

We allow string[] to be assigned to object[], and so we also allow string[] to be assigned to string?[].

>
> public void M6(string?[] p)
> {
> string[] v1 = p; // Warning
> string[] v2 = p!; // No warning
> }
> }
> ```
>
> *end example*

Types that are invariant likewise may produce a warning when the conversion differs only in the nullability of type arguments.

> <!-- Example: {template:"code-in-class-lib", name:"NullInvariance"} -->
> ```csharp
> #nullable enable
> public class C
> {
> public void M1(List<string> p)
> {
> List<string?> v1 = p; // Warning
> List<string?> v1 = p!; // No Warning
> }
>
> public void M2(List<string?> p)
> {
> List<string> v1 = p; // Warning
> List<string> v1 = p!; // No Warning
> }
> }
> ```
>
> *end example*

***End of conditionally normative text***
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