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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 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).

// cs_operator_addition_assignment.cs
using System;
class MainClass
    static void Main() 
        int a = 5;
        a += 6;

        //string concatenation
        string s = "Micro";
        s += "soft";