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+= 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 depends on the types of x and y (addition for numeric operands, concatenation for string operands, and so forth).
The += operator is also used to specify a method that will be called in response to an event; such methods are called event handlers. The use of the += operator in this context is referred to as subscribing to an event. For more information, see How to: Subscribe to and Unsubscribe from Events (C# Programming Guide). and Delegates (C# Programming Guide).