Custom event handler called twice?

Question

I created an event handler that simply returns a list of objects that I get from the web service when the call ends.

Now I went ahead and launched the application in debug mode and found out that it works fine on the first call, but immediately after its completion, the event is fired a second time. I checked and am absolutely sure that I do not fire the event more than once in the receiver class.

This is my first snapshot of creating custom event handlers inside my applications, so I'm not quite sure that the implementation is 100% accurate.

Any ideas on what could be causing this? Is the way I created the event handler accurate?

This is the DataHelper class

public class DataHelper
{
    public delegate void DataCalledEventHandler(object sender, List<DataItem> dateItemList);
    public event DataCalledEventHandler DataCalled;

    public DataHelper()
    {

    }

    public void CallData()
    {
        List<DataItem> dataItems = new List<DataItem>();
        //SOME CODE THAT RETURNS DATA
        DataCalled(this, dataItems);
    }
}

Here I signed up for my event:

protected override void OnNavigatedTo(System.Windows.Navigation.NavigationEventArgs e)
{
   GetNewDataItems();
}
private void GetNewDataItems()
        {

                try
                {
                    DataHelper dataHelper = new DataHelper();
                    dataHelper.CallData();
                    dataHelper.DataCalled += new DataHelper.DataCalledEventHandler(dataHelper_DataCalled);

                }
                catch
                {
                   //Handle any errors
                }
            }
    }

    void dataHelper_DataCalled(object sender, List<DataItem> dataItemsList)
    {
        //Do something with results
        //NOTE: THIS IS WHERE THE EXCEPTION OCCURS WHEN EVENT IS FIRED FOR SECOND TIME
    }

Answer 1

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, - ...

private Class1 instance1;

void callback(...)
{
}

void myfunction()
{
    this.instance1.DataCalled += this.callback;
    this.instance1.DataCalled += this.callback;
}

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null , , NullReferenceException. , .

public void CallData()
{
    List<DataItem> dataItems = new List<DataItem>();
    var handler = this.DataCalled;
    if (handler != null)
        handler(this, dataItems);
}

: , , , , GetNewDataItems, . , , , , - , .

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- ...

void dataHelper_DataCalled(object sender, List<DataItem> dataItemsList)
{
    // Deregister the event...
    (sender as Class1).DataCalled -= dataHelper_DataCalled; 

    //Do something with results
}

, , , .

, , . , null, .

// in data helper class

private DataHelper.DataCalledEventHandler myFunctor;

public void CallData(DataHelper.DataCalledEventHandler functor)
{
    this.myFunctor = functor;
    //SOME CODE THAT RETURNS DATA
}

// when the call completes, asynchronously...
private void WhenTheCallCompletes()
{
    var functor = this.myFunctor;
    if (functor != null)
    {
        this.myFunctor = null;
        List<DataItem> dataItems = new List<DataItem>();
        functor(this, dataItems);
    }
}
    
// in your function
...    dataHelper.CallData(this.dataHelper_DataCalled);    ...

Answer 2

.

dataHelper.CallData();
dataHelper.DataCalled += new DataHelper.DataCalledEventHandler(dataHelper_DataCalled);

:

dataHelper.DataCalled += new DataHelper.DataCalledEventHandler(dataHelper_DataCalled);
dataHelper.CallData();

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