Delegate and Null

Question

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.

Vulpes · 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 Marla · 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 LoggerLogger2 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.

hocine cheniki · Answer

thank you very much i feel good now

Sukesh Marla · Answer

always welcome