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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 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).

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
*/

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