Delegate and Null

hocine cheniki

hi all ,
i have a bit misunderstand with the delegate
the first one is why we have the verify if the delegate null or no ?
public class MyClass
{
// Declare a delegate that takes a single string parameter
// and has no return type.
public delegate void LogHandler(string message);

// The use of the delegate is just like calling a function directly,
// though we need to add a check to see if the delegate is null
// (that is, not pointing to a function) before calling the function.
public void Process(LogHandler logHandler)
{
if (logHandler != null)
{
logHandler("Process() begin");
}

if (logHandler != null)
{
logHandler ("Process() end");
}
}
}

the second one is why we have to refer the delegate with null and sometimes no?
like this:
MyClass.LogHandler myLogger = null;
myLogger += new MyClass.LogHandler(Logger);

vs this:
MyClass.LogHandler myLogger = new MyClass.LogHandler(Logger);

please help me i can't understand those collision between them.

4 Replies

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Vulpes14y ago
Accepted answer
Delegates are reference types and so have a default value of null.

However, as soon as you add a method to their invocation list, the delegate is instantiated.

So, before attempting to invoke a delegate, you should first check whether it's null or not. There are two reasons for this:

1. If the delegate is null, there will be no methods to invoke.

2. If you attempt to invoke a null delegate, you'll get a null reference exception.

On the second point, as Sukesh has already said, the two pieces of code are equivalent to each other i.e.

myLogger += new MyClass.LogHandler(Logger);

is equivalent, when myLogger is null, to:

myLogger = new MyClass.LogHandler(Logger);
Sukesh Marla14y ago
Accepted answer
When u use =, it means you are creating Single cast delegate, and += means MultiCast

Example
MyClass.LogHandler myLogger = null;
myLogger = new MyClass.LogHandler(Logger); //Delegate point to Logger Method

myLogger += new MyClass.LogHandler(Logger2);//Now delegate point to Logger and Logger2

myLogger += new MyClass.LogHandler(Logger3);//Now delegate point to Logger and Logger2 and Logger3

when u invoke myLogger Logger,Logger2 and Logger3 All will exeucted one after other.

Just like variable,
int i=1;
i+=4;// i is 5
i+=3;// i is 8

now if you say i=3 // i is 3

Similar way after adding Logger2 and Logger3
if you say myLogger=null; // now myLogger is not pointing to anything

now += acts as first +

in Short
MyClass.LogHandler myLogger = null;
myLogger += new MyClass.LogHandler(Logger);
and
MyClass.LogHandler myLogger = new MyClass.LogHandler(Logger);
Both are same//Second one will be better if you want to create single cast delegate

Hopw you understood

Check this is correct answer if it helped.
Sukesh Marla14y ago
always welcome
hocine cheniki14y ago
thank you very much ,i feel good now