CODEWITHSUNDEEP
c#eventsevent-handling
vsh
vsh

Reputation: 53

The event is always null in the scenario given below

There are 3 classes A, B, C(Consumer). Class A calls B to fire event so that Class C can receive as it has subscribed. How to achieve this functionality?

Below is the code.

  public delegate void TestDelegate();
   public class B
 {
    public event TestDelegate TestEvent;

    public B()
    {

    }


    public void Fire()
    {
        TestEvent();//Null reference exception as not subscribed to the event as TestEvent is always null
    }
}
public class A
{
        static void Main()
{
 B b = new B();
b.Fire(); //Null reference exception as not subscribed to the event.
}
}

//Consumer application
public Class C
 {
  static void Main()
 {
 B b = new B();
 b.TestEvent+= new TestDelegate(c_TestEvent);

 }

    static void c_TestEvent()
      {
         Console.WriteLine("Console 2 Fired");
     }

 }

Upvotes: 1

Views: 45

Answers (3)

C. McCoy IV
C. McCoy IV

Reputation: 897

Here's how to do it if you use the same instance of B:

using System;

namespace StackOverflow_Events
{
    class Program
    {
        static void Main(string[] args)
        {
            B b = new B();
            A a = new A(b);
            C c = new C(b);

            a.Test();

            Console.ReadKey();
        }
    }

    public delegate void TestDelegate();

    public class B
    {
        public event TestDelegate TestEvent;

        public B()
        {
        }

        public void Fire()
        {
            TestEvent?.Invoke();
        }
    }

    public class A
    {
        private B b;

        public A(B _b)
        {
            b = _b;
        }

        public void Test()
        {
            b.Fire(); 
        }
    }

    //Consumer application
    public class C
    {
        private B b;

        public C(B _b)
        {
            b = _b;
            b.TestEvent += new TestDelegate(c_TestEvent);
        }

        static void c_TestEvent()
        {
            Console.WriteLine("Console 2 Fired");
        }
    }

}

Here's how to do it statically:

using System;

namespace StackOverflow_Events
{
    class Program
    {
        static void Main(string[] args)
        {
            C.Init();

            A.Test();

            Console.ReadKey();
        }
    }

    public delegate void TestDelegate();

    public class B
    {
        public static event TestDelegate TestEvent;

        public B()
        {
        }

        public void Fire()
        {
            TestEvent?.Invoke();
        }
    }

    public class A
    {
        public static void Test()
        {
            B b = new B();
            b.Fire(); 
        }
    }

    //Consumer application
    public class C
    {
        public static void Init()
        {
            B.TestEvent += new TestDelegate(c_TestEvent);
        }

        static void c_TestEvent()
        {
            Console.WriteLine("Console 2 Fired");
        }
    }

}

Upvotes: 0

Jamiec
Jamiec

Reputation: 136164

Just ensure TestEvent is not null

public void Fire()
{
    if(TestEvent != null)
        TestEvent();
}

The newer way to do this using the safe navigation operator ?.

public void Fire()
{
    TestEvent?.Invoke();
}

Upvotes: 1

Gururaj
Gururaj

Reputation: 539

There are multiple issues with your code

  • There are more than one Main (entry point). Don't know which one is taking precedence
  • There are more than one instance of B. The instance in A is firing the event whereas it does not have any event handler associated.

Refactor your code such that there is only one entry point into your application and also have one instance of B which will be used to subscribe to the event.

Upvotes: 0

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