+= 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 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).

Example

// cs_operator_addition_assignment.cs
using System;
class MainClass
{
    static void Main() 
    {
        int a = 5;
        a += 6;
        Console.WriteLine(a);
        string s = "Micro";
        s += "soft";
        Console.WriteLine(s);
    }
}

Output

11
Microsoft

See Also

Reference

C# Operators

Concepts

C# Programming Guide

Other Resources

C# Reference