Why cant i overload the += operator in C#? But i s

Question

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Simple way to overload compound assignment operator in C#?

I was playing around with events and thought events are weird. Why couldnt i just implement them in a generic class. So i tried to and found that i CANT OVERLOAD +=. From the language specs found here

The overloadable unary operators are:    
+   -   !   ~   ++   --   true   false

The overloadable binary operators are:    
+   -   *   /   %   &   |   ^   <<   >>   ==   !=   >   <   >=   <=

+= is not listed. Now, before you say theres no reason to overload += i'd like to bring up the fact C# has events which uses the += operator and the fact i tried to implement an event for fun and wanted to use the += operator. Now, i have a feeling someone will say thats why events exist, because it is the only reason. However i want to bring up you can use += with the TimeSpan struct. Go try it, var ts= new TimeSpan(); ts += ts; will compile and run.

I looked at TimeSpan definition and i have no idea how it is allowing it. I saw a public static TimeSpan operator +(TimeSpan t); which looked suspicious but then i realize its for something like var name = +ts; like how you can do var name = -ts; for negation.

So my question is, how do i use the += for my struct or class. It appears to be supported i just cant seem to find the documentation on it.

Answer 1

The documentation doesn't lie. It's not possible to overload the += operator.

The += operator is just a shortcut for using the + operator. An operation like x += y is translated into x = x + y by the compiler.

So, to change what the += operator does, you have to overload the + operator.

Answer 2

+= is just a shorthand for addition, then assignment. You can overload the + operator, which also makes the += operator work:

This compiles

class P 
{
    public static P operator +(P rhs, P lhs) {
        return null;
    }   
}

void Main()
{
    P p = new P();
    p += new P();
}

(And, vice versa for - and -= operators, of course).

Answer 3

Events don't overload the += and -= operator. This is just hardcoded in the C# compiler which translates it into the add/remove methods of that event.

You can only overload + and - which then indirectly overload += and -=. If you overload + then automatically x += y becomes overloaded to x = x + y. So for your type you don't need to put in any extra work. Just overload + and you'll get += for free. This of course requires the left side argument and the result type being compatible, but that's usually the case.

---

The reason that you can't overload += separately is most likely that it creates strange semantics for reference types. If += modified the existing instance of the left side then the following code would behave strangely:

MyClass x=new MyClass();
MyClass y=x;
x+=SomeThing;
y has changed now too

On the other hand with += meaning x = x+y this code creates a new instance which it assigns to x. And the original instance remains unmodified in y;

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In C++ on the other hand it is possible to safely specify a separate overload for += because you can overload the assignment and copy-constructor at the same time and use that to get the correct semantics.

Answer 4

The += operator cannot be overloaded directly, but user-defined types can overload the + operator.

An expression using the += assignment operator, such as Copy

x += y

is equivalent to Copy

x = x + y

Msdn