+= Operator (C# Reference)
The addition assignment operator.
An expression using the += assignment operator, such as
x += y
is equivalent to
x = x + y
except that x is only evaluated once. The meaning of the + operator is dependent on the types of x and y (addition for numeric operands, concatenation for string operands, and so forth).
The += operator cannot be overloaded directly, but user-defined types can overload the + operator (see operator).
The += operator is also used to specify a method that will be called in response to an event; such methods are called event handlers. Since event handlers are encapsulated within a delegate type, the use of the += operator in this context is referred to as delegate concatenation. For more information, see event (C# Reference) and Delegates (C# Programming Guide).