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Apr 16, 2025
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Add notnull example #1309

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Add notnull example
jskeet Apr 15, 2025
31c66e2
Adjust wording
jskeet Apr 16, 2025
7709197
It is => it shall be
jskeet Apr 16, 2025
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35 changes: 34 additions & 1 deletion standard/classes.md
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Expand Up @@ -487,7 +487,40 @@ For a type parameter `T` when the type argument is a nullable reference type `C?

The ***not null*** constraint specifies that a type argument used for the type parameter should be a non-nullable value type or a non-nullable reference type. A type argument that isn’t a non-nullable value type or a non-nullable reference type is allowed, but a compiler may produce a diagnostic warning.

Because `notnull` is not a keyword, in *primary_constraint* the not null constraint is always syntactically ambiguous with *class_type*. For compatibility reasons, if a name lookup ([§12.8.4](expressions.md#1284-simple-names)) of the name `notnull` succeeds it is treated as a `class_type`. Otherwise it is treated as the not null constraint.
Because `notnull` is not a keyword, in *primary_constraint* the not null constraint is always syntactically ambiguous with *class_type*. For compatibility reasons, if a name lookup ([§12.8.4](expressions.md#1284-simple-names)) of the name `notnull` succeeds it shall be treated as a `class_type`. Otherwise it shall be treated as the not null constraint.

> *Example*: The following class demonstrates the use of various type arguments against different constraints, indicating warnings which may be issued by a compiler.
>
> <!-- Example: {template:"standalone-lib-without-using", name:"TypeParameterConstraints0", expectedWarnings:["CS8714","CS8714","CS8631"], ignoredWarnings:["CS0168"]} -->
> ```csharp
> #nullable enable
> public class C { }
> public class A<T> where T : notnull { }
> public class B1<T> where T : C { }
> public class B2<T> where T : C? { }
> class Test
> {
> static void M()
> {
> // nonnull constraint allows nonnullable struct type argument
> A<int> x1;
> // possible warning: nonnull constraint prohibits nullable struct type argument
> A<int?> x2;
> // nonnull constraint allows nonnullable class type argument
> A<C> x3;
> // possible warning: nonnull constraint prohibits nullable class type argument
> A<C?> x4;
> // nonnullable base class requirement allows nonnullable class type argument
> B1<C> x5;
> // possible warning: nonnullable base class requirement prohibits nullable class type argument
> B1<C?> x6;
> // nullable base class requirement allows nonnullable class type argument
> B2<C> x7;
> // nullable base class requirement allows nullable class type argument
> B2<C?> x8;
> }
> }
> ```

The value type constraint specifies that a type argument used for the type parameter shall be a non-nullable value type. All non-nullable struct types, enum types, and type parameters having the value type constraint satisfy this constraint. Note that although classified as a value type, a nullable value type ([§8.3.12](types.md#8312-nullable-value-types)) does not satisfy the value type constraint. A type parameter having the value type constraint shall not also have the *constructor_constraint*, although it may be used as a type argument for another type parameter with a *constructor_constraint*.

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