77 lines
2.5 KiB
Markdown
77 lines
2.5 KiB
Markdown
# Declaration expressions
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Support declaration assignments as expressions.
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## Motivation
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[motivation]: #motivation
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Allow initialization at the point of declaration in more cases, simplifying code, and allowing `var` to be used.
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```csharp
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SpecialType ReferenceType =>
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(var st = _type.SpecialType).IsValueType() ? SpecialType.None : st;
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```
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Allow declarations for `ref` arguments, similar to `out var`.
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```csharp
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Convert(source, destination, ref List<Diagnostic> diagnostics = null);
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```
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## Detailed design
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[design]: #detailed-design
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Expressions are extended to include declaration assignment. Precedence is the same as assignment.
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```antlr
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expression
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: non_assignment_expression
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| assignment
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| declaration_assignment_expression // new
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;
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declaration_assignment_expression // new
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: declaration_expression '=' local_variable_initializer
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;
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declaration_expression // C# 7.0
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| type variable_designation
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;
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```
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The declaration assignment is of a single local.
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The type of a declaration assignment expression is the type of the declaration.
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If the type is `var`, the inferred type is the type of the initializing expression.
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The declaration assignment expression may be an l-value, for `ref` argument values in particular.
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If the declaration assignment expression declares a value type, and the expression is an r-value, the value of
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the expression is a copy.
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The declaration assignment expression may declare a `ref` local.
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There is an ambiguity when `ref` is used for a declaration expression in a `ref` argument.
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The local variable initializer determines whether the declaration is a `ref` local.
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```csharp
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F(ref int x = IntFunc()); // int x;
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F(ref int y = RefIntFunc()); // ref int y;
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```
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The scope of locals declared in declaration assignment expressions is the same the scope of corresponding declaration expressions from C#7.0.
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It is a compile time error to refer to a local in text preceding the declaration expression.
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## Alternatives
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[alternatives]: #alternatives
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No change. This feature is just syntactic shorthand after all.
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More general sequence expressions: see [#377](https://github.com/dotnet/csharplang/issues/377).
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To allow use of `var` in more cases, allow separate declaration and assignment of `var` locals,
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and infer the type from assignments from all code paths.
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## See also
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[see-also]: #see-also
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See Basic Declaration Expression in [#595](https://github.com/dotnet/csharplang/issues/595).
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See Deconstruction Declaration in the [deconstruction](https://github.com/dotnet/roslyn/blob/master/docs/features/deconstruction.md) feature.
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