+= Operator (C# Reference)
The addition assignment operator.
Remarks
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 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. 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).
Example
class AddAssigment
{
static void Main()
{
//addition
int a = 5;
a += 6;
Console.WriteLine(a);
//string concatenation
string s = "Hello";
s += " world.";
Console.WriteLine(s);
}
}
/*
Output:
11
Hello world
*/