C# Language Specification

Event assignment

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 (§6.1).
  
• 
  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 (§8.6).

18.