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5874493 Jun 6, 2017
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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.

A throw expression throws the value produced by evaluating the null_coalescing_expression, which must denote a value of the class type System.Exception, of a class type that derives from System.Exception or of a type parameter type that has System.Exception (or a subclass thereof) as its effective base class. If evaluation of the expression produces null, a System.NullReferenceException is thrown instead.

The behavior at runtime of the evaluation of a throw expression is the same as specified for a throw statement.

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.