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C# 7.x: Add support for readonly structs #333

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Merged
merged 3 commits into from
Oct 7, 2022
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C# 7.x: Add support for readonly structs #333

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0569b1d
Update structs.md
RexJaeschke Jun 15, 2021
6f6a6af
lint fixes
BillWagner Apr 3, 2022
739d370
Moved `ref struct` specification to https://github.com/dotnet/csharps…
Jul 19, 2022
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17 changes: 14 additions & 3 deletions standard/structs.md
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Expand Up @@ -16,8 +16,8 @@ A *struct_declaration* is a *type_declaration* ([§13.7](namespaces.md#137-type-

```ANTLR
struct_declaration
: attributes? struct_modifier* 'partial'? 'struct' identifier
type_parameter_list? struct_interfaces?
: attributes? struct_modifier* 'partial'? 'struct'
identifier type_parameter_list? struct_interfaces?
type_parameter_constraints_clause* struct_body ';'?
;
```
Expand All @@ -39,6 +39,7 @@ struct_modifier
| 'protected'
| 'internal'
| 'private'
| 'readonly'
| unsafe_modifier // unsafe code support
;
```
Expand All @@ -47,7 +48,17 @@ struct_modifier

It is a compile-time error for the same modifier to appear multiple times in a struct declaration.

The modifiers of a struct declaration have the same meaning as those of a class declaration ([§14.2.2](classes.md#1422-class-modifiers)).
Except for `readonly`, the modifiers of a struct declaration have the same meaning as those of a class declaration ([§14.2.2](classes.md#1422-class-modifiers)).

The `readonly` modifier indicates that the *struct_declaration* declares a type whose instances are immutable.

A readonly struct has the following constraints:

- Each of its instance fields shall also be declared `readonly`.
- None of its instance properties shall have a *set_accessor_declaration* ([§14.7.3](classes.md#1473-accessors)).
- It shall not declare any field-like events ([§14.8.2](classes.md#1482-field-like-events)).

When an instance of a readonly struct is passed to a method, its `this` is treated like an `in` argument/parameter, which disallows write access to any instance fields (except by constructors).

### 15.2.3 Partial modifier

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