csharplang/proposals/csharp-7.0/throw-expression.md

Throw expression

We extend the set of expression forms to include

throw_expression
    : 'throw' null_coalescing_expression
    ;

null_coalescing_expression
    : throw_expression
    ;

The type rules are as follows:

• A throw_expression has no type.
• A throw_expression is convertible to every type by an implicit conversion.

The flow-analysis rules are as follows:

• For every variable v, v is definitely assigned before the null_coalescing_expression of a throw_expression iff it is definitely assigned before the throw_expression.
• For every variable v, v is definitely assigned after throw_expression.

A throw expression is permitted in only the following syntactic contexts:

• As the second or third operand of a ternary conditional operator ?:
• As the second operand of a null coalescing operator ??
• As the body of an expression-bodied lambda or method.

Note: the rest of the semantics of the throw expression are identical to the semantics of the throw_statement in the current language specification.