Monday, December 31, 2012

Passing Data Between Forms Redux

Almost exactly two years ago, I published the Passing Data Between Forms post. I have had numerous questions and comments about that post and now I’ve decided that it deserves an additional example.

In that post, I had an example (the 3rd one) that utilized Interfaces. In the example, we had a MainForm that passed a DataSet to Form1. Form1 then allowed the user to make some changes to the data in that DataSet and then we wanted to have MainForm automatically reflect those changes ... even while the user was still working in Form1. An interface worked well for that scenario. At the time, I wanted to keep the post simple so as not to confuse beginners. But, I really need to expand on that example and show how to accomplish this task using delegates/events.

So, without further ado (pun intended), here it is:

First, let's take a look at Form1. Minimally, you can implement this whole thing with a simple event, which uses standard EventArgs, as follows:

public class Form1 : Form
{
    public event EventHandler DataChanged;
    private CustomerDataSet oData;

    public Form1(CustomerDataSet dsCust)
    {
        this.oData = dsCust;
    }
    public void DoStuff()
    {
        // code to do stuff with this.oData
        // ...
        // and then fire the event if anyone has subscribed
        this.OnDataChanged(new EventArgs());
    }
    private void OnDataChanged(EventArgs e)
    {
        if (this.DataChanged != null)
            this.DataChanged(this, e);
    }
}

Then, the MainForm looks like this:

public class MainForm : Form
{
    private CustomerDataSet dsCustomer;

    // ...

    // then code elsewhere to instantiate and fill your data
    this.dsCustomer = new CustomerDataSet();
    // plus maybe other code to fill the dataset


    // ...

    // code to instantiate Form1, pass it the DataSet, handle the event
    Form1 oForm = new Form1(this.dsCustomer);
    oForm.DataChanged += new EventHandler(DataChanged);
    oForm.Show();

    public void DataChanged(object sender, EventArgs e)
    {
        // code here to do stuff with this.dsCustomer
    }
}

In this particular case, we don't really need EventArgs. Because we passed the DataSet into Form1 to begin with, we already know everything we need to know, namely the data that has changed will be the same DataSet that we passed into Form1. So, since we don’t need any EventArgs, we could create a custom delegate to use as our event handler. We will change our two forms like this:

Form1:

public class Form1 : Form
{
    public delegate void DataChangedEventHandler();
    public DataChangedEventHandler DataChanged;
    private CustomerDataSet oData;

    public Form1(CustomerDataSet dsCust)
    {
        this.oData = dsCust;
    }
    public void DoStuff()
    {
        // code to do stuff with this.oData
        // ...
        // and then fire the event if anyone has subscribed
        this.OnDataChanged();
    }
    private void OnDataChanged()
    {
        if (this.DataChanged != null)
            this.DataChanged();
    }
}

The only difference in MainForm is that the event handler doesn't need the usual parameters (object sender, EventArgs e), so change the DataChanged event handler to look like this:

// code to instantiate Form1, pass it the DataSet, handle the event
Form1 oForm = new Form1(this.dsCustomer);
oForm.DataChanged += new Form1.DataChangedEventHandler(DataChanged);
oForm.Show();

public void DataChanged()
{
    // code here to do stuff with this.dsCustomer
}

Another way of handling the event in the MainForm is to not even bother with the above DataChanged() method and use an anonymous delegate instead, so you could change the MainForm code to look like this:

// code to instantiate Form1, pass it the DataSet, handle the event
Form1 oForm = new Form1(this.dsCustomer);
oForm.DataChanged += delegate
{ 
    // code here to do stuff with this.dsCustomer
};
oForm.Show();

There are other things you can do when creating your own delegates and/or events and, in fact, I have a written a blog post about a DataAccess class that does some interesting things with anonymous delegates: DataAccess - Part III.

However, expanding on ideas for delegates and/or events is beyond the scope of this simple post. Perhaps in the future I’ll write something more extensive, but in the meantime, you can always start off with an MSDN article such as this one: Handling And Raising Events

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