40 lines
1.8 KiB
Markdown
40 lines
1.8 KiB
Markdown
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Out Variable Declarations
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=========================
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The *out variable declaration* feature enables a variable to be declared at the location that it is being passed as an `out` argument.
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```antlr
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argument_value
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: 'out' type identifier
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| ...
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;
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```
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A variable declared this way is called an *out variable*. You may use the contextual keyword `var` for the variable's type. The scope will be the same as for a *pattern-variable* introduced via pattern-matching.
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According to Language Specification (section 7.6.7 Element access) the argument-list of an element-access (indexing expression) does not contain ref or out arguments. However, they are permitted by the compiler for various scenarios, for example indexers declared in metadata that accept `out`.
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Within the scope of a local variable introduced by an argument_value, it is a compile-time error to refer to that local variable in a textual position that precedes its declaration.
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It is also an error to reference an implicitly-typed (§8.5.1) out variable in the same argument list that immediately contains its declaration.
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Overload resolution is modified as follows:
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We add a new conversion:
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> There is a *conversion from expression* from an implicitly-typed out variable declaration to every type.
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Also
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> The type of an explicitly-typed out variable argument is the declared type.
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and
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> An implicitly-typed out variable argument has no type.
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The *conversion from expression* from an implicitly-typed out variable declaration is not considered better than any other *conversion from expression*.
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The type of an implicitly-typed out variable is the type of the corresponding parameter in the signature of the method selected by overload resolution.
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The new syntax node `DeclarationExpressionSyntax` is added to represent the declaration in an out var argument.
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