PLS HELP: Using events in inherited classes

[MuZZy]

Hi,

Consider i have this code:
// ---------------------------------------------------------------------
public delegate void DoSearchEventHandler(Form f, DoSearchEventArgs e);
public class DoSearchEventArgs
{
public String m_sIDColumn;
public String m_sQuery;
public DoSearchEventArgs(String sQuery, String sIDColumn)
{
m_sQuery = sQuery;
m_sIDColumn = sIDColumn;
}
}
public class SearchForm: Form
{
public event DoSearchEventHandler DoSearchEvent;
public bool DoSearchEventEmpty
{
get { return DoSearchEvent == null; }
}
public void DoSearchEventFire(Form f, DoSearchEventArgs e)
{
DoSearchEvent(f, e);
}
}
//----------------------------------------------------------------------
Now i have my form inherited from SearchForm and in some func of this
new form i want to use that event "DoSearchEvent":

public class MyForm: SearchForm
{
public void Search()
{
if (DoSearchEvent != null)
DoSearchEvent(this, new DoSearchEventArgs("",""))

}
}

Compiler gives an error on line "if (DoSearchEvent != null)" that you
can only use += or -= for events outside the class where they were
declared.
But MyClass is inherited from SearchClass so why events don't get
inherited as well?

Currently i have to declare two functions for each of those events in
SeachForm class: SetEvent and FireEvent, so for 10 events i have 20
pretty much redundant functions which differ by sematics but same by sence.

What should be the best way out?


Thank you in advance,
MuZZy

 

[Greg Young]

The best way of handling this is to put a protected method that raises the
event in the base. The derived class can then call that method which tells
the base to raise the event.

Cheers,

Greg Young
MVP - C#
http://codebetter.com/blogs/gregyoung/

 

[Linda Liu [MSFT]]

Hi MuZZy,

Thank you for posting.

If a base class contains an event variable, you can access it in a derived
class directly without using any object instance as a prefix. However, you
must use += or -= for the event variable outside the base class, which even
includes the derived class. It's a syntax.

I think you could define a "common" function to fire the events which have
the same signature in the base class. You can use a Dictionary to store the
event instances. In the common function to fire the events, you should get
the delegate instance out from the Dictionary and fire the event.

Here is a sample as follows.

public delegate void Delegate1(int i);

public class PropertyEventsSample
{
private System.Collections.Generic.Dictionary<string,
System.Delegate> eventTable;

public PropertyEventsSample()
{
eventTable = new
System.Collections.Generic.Dictionary<string, System.Delegate>();
eventTable.Add("Event1", null);
eventTable.Add("Event2", null);
}

public event Delegate1 Event1
{
add
{
eventTable["Event1"] = (Delegate1)eventTable["Event1"]
+ value;
}
remove
{
eventTable["Event1"] = (Delegate1)eventTable["Event1"]
- value;
}
}

public event Delegate1 Event2
{
add
{
eventTable["Event2"] = (Delegate1)eventTable["Event2"]
+ value;
}
remove
{
eventTable["Event2"] = (Delegate1)eventTable["Event2"]
- value;
}
}

// this is the common function to fire the two events
internal void FireDelegate1Event(string eventname,int i)
{
Delegate1 D;
if (null != (D = (Delegate1)eventTable[eventname]))
{
D(i);
}
}
}

public class TestClass
{
public static void Delegate1Method(int i)
{
System.Console.WriteLine(i);
}

static void Main()
{
PropertyEventsSample p = new PropertyEventsSample();

p.Event1 += new Delegate1(TestClass.Delegate1Method);

p.FireDelegate1Event("Event1",2);

p.Event2 += new Delegate1(TestClass.Delegate1Method);

p.FireDelegate1Event("Event2",4);

Console.ReadLine();
}
}

Hope this is helpful to you.
If you have any other concerns or need anything else, please don't hesitate
to let me know.



Sincerely,
Linda Liu
Microsoft Online Community Support

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