If the left operand of a += or -= operator is classified as an event access, then the expression is evaluated as follows:

• The instance expression, if any, of the event access is evaluated.
• The right operand of the += or -= operator is evaluated, and, if required, converted to the type of the left operand through an implicit conversion (13.1).
• An event accessor of the event is invoked, with argument list consisting of the right operand, after evaluation and, if necessary, conversion. If the operator was +=, the add accessor is invoked; if the operator was -=, the remove accessor is invoked.

An event assignment expression does not yield a value. Thus, an event assignment expression is valid only in the context of a statement-expression (15.6).