Dun understand why Add when it should be Substrate

Tangara G

9y733Learn iOS Programming

Dear experts,

I have this multi-cast delegate which I am having problem understand:

delegate void M(int x, int y);
public class Oper
{
public static void Add(int x, int y)
{
Console.WriteLine("{0} + {1} = {2}", x, y, x+y);
}
public static void Sub(int x, int y)
{
Console.WriteLine("{0} + {1} = {2}", x, y, x+y);
}
public class CSharpApp
{
static void Main()
{
M del = new M(Oper.Add);
del += new M(Oper.Sub);
del(6,4);
del -= new M(Oper.Sub);
del(2,8);
}
}

I would like to know why the output for del(2,8) is 10 instead of 2 - 8 which is -6.

7 Replies

Know the answer? Post it — somebody with the same question will find it here.

Nitin Sontakke9y ago
Accepted answer
Yes, that's right. If you read the original article, you will notice same output there as well.
Tangara G9y ago
Tks Nitin.

Let me try out when my VS is ready here. if I really want to use the Minus method...i guess I should use del+= the Sub Method...

Hv a nice day.
Tangara G9y ago
Hi Nitin,

Just to check my understanding :

del -= new M(Oper.Sub) will remove the Sub Method and so now it is left with the Add Method hence plus ?

Kindly confirm.

Tk you.
Nitin Sontakke9y ago
Hope you have changed the sign in double quotes as well!
public static void Sub(int x, int y)
{
Console.WriteLine("{0} - {1} = {2}", x, y, x-y);
}
Nitin Sontakke9y ago
Can you please paste the complete output (all Console.WriteLine printed) that you are getting? May be then I will be able to make some educated guess!

I think, you might be getting:
10
2
10

If yes, notice that before first call to "del" there are two functions to call, First add then substract. You call it with 6 and 4 so you get 10 (6+4) and 2 (6-4). Then from "del" you unregister the call to Sub function, with the line del -= new M(Oper.Sub);

Now "del" has left with only one function call that is "add". So when you call it with 2 and 8, you get 10. Hope you get it.
Tangara G9y ago
Hi Nitin,

You have sharp eye !

I did a comparision with the original code at zetcode http://zetcode.com/lang/csharp/delegates/ . So this is my mistake.

But, the output is still 10 instead of -6 with the correct line of code that you just mentioned. How come it is so?
Nitin Sontakke9y ago
Because in Sub method you haven't changed the sign from + to -.

It should be:
public static void Sub(int x, int y)
{
Console.WriteLine("{0} + {1} = {2}", x, y, x-y);
}