call an eventhandler with arguments

Question

Visual Studio 2008, C# 3.0.

I have a method below which calls an event handler. I would like to pass the two arguments received by the method to the event handler.

I would like to do something like this:

wc.DownloadDataCompleted += wc.DownloadedDataCompleted(strtitle, placeid);

Is this even possible, if yes, how would I go about doing it ?

Code Snippet:

public void downloadphoto(string struri,string strtitle,string placeid)
{
    using (WebClient wc = new WebClient())
    {
        wc.DownloadDataCompleted += wc_DownloadDataCompleted;
        wc.DownloadDataAsync(new Uri(struri));
    }
}

score 25 · Accepted Answer · answered Sep 28 '09 at 16:54

25

The easiest way to do this is to use an anonymous function (an anonymous method or a lambda expression) to subscribe to the event, then make your method have just the parameters you want:

public void downloadphoto(string struri, string strtitle, string placeid)
{
    using (WebClient wc = new WebClient())
    {
        wc.DownloadDataCompleted += (sender, args) => 
            DownloadDataCompleted(strtitle, placeid, args);
        wc.DownloadDataAsync(new Uri(struri));
    }
}

// Please rename the method to say what it does rather than where it's used :)
private void DownloadDataCompleted(string title, string id, 
                                   DownloadDataCompletedEventArgs args)
{
    // Do stuff here
}

answered Sep 28 '09 at 16:54

Jon Skeet

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1

Awesome !! now who would have thunk that !! – dezkev Sep 29 '09 at 09:55
3

How do you unsubscribe to the event? – jdelator Apr 05 '12 at 01:36
2

@jdelator: You would need to store the delegate in a variable and use that to unsubscribe. – Jon Skeet Apr 05 '12 at 05:50

score 5 · Answer 2 · answered Sep 28 '09 at 16:54

5

DownloadDataAsync has an overload which takes an object:

DownloadDataAsync(uri, object)

That object can be any arbitrary thing you want that gets passed into the DownloadDataCompleted handler:

public void downloadphoto(string struri,string strtitle,string placeid)
{
    using (WebClient wc = new WebClient())
    {
        string[] data = new string[2] { strtitle, placeid };
        wc.DownloadDataCompleted += wc_DownloadDataCompleted;
        wc.DownloadDataAsync(new Uri(struri), data);
    }
}


void wc_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
{
    string[] data = (string[])e.UserToken;
    string strtitle = data[0];
    string placeid = data[1];
}

answered Sep 28 '09 at 16:54

Rex M

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283
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Jon's version is funky, but this works and is compatible with non-3.5 stuff. – womp Sep 28 '09 at 16:59
@womp Jon's version is ~~funky~~ awesome, but this works and is compatible with non-3.5 stuff :) – Rex M Sep 28 '09 at 17:01
So we agree. It's funky awesome. – womp Sep 28 '09 at 17:10
Just to clarify: my version needs C# 3, but not .NET 3.5. To do it with C# 2 you'd just use an anonymous method instead of a lambda expression. – Jon Skeet Sep 29 '09 at 09:58
I needed to use e.UserState as there was no UserToken in DownloadDataCompletedEventArgs – p10ben Jan 19 '16 at 05:01

JDunkerley · Answer 3 · 2009-09-28T17:01:22.057

You could create a private class and place the handler in there. E.g.

    public void downloadphoto(string struri, string strtitle, string placeid)
    {
        using (WebClient wc = new WebClient())
        {
            wcHandler handler = new wcHandler() { Strtitle = strtitle, Placeid = placeid };
            wc.DownloadDataCompleted += handler.wc_DownloadDataCompleted;
            wc.DownloadDataAsync(new Uri(struri));
        }

    }

    private class wcHandler
    {
        public string Strtitle { get; set; }
        public string Placeid { get; set; }

        public void wc_DownloadDataCompleted(object sender, DownloadDataCompletedEventArgs e)
        {
            // Do Stuff
        }
    }

Although, given the elegance of Jon's answer would probably use that!

score 1 · Answer 4 · answered Dec 27 '12 at 22:19

Jon Skeet already answered this, showing how to use a lamda expression, but I was still unclear about it. I still needed some more examples, and eventually found this simple case using a button: http://social.msdn.microsoft.com/Forums/en-US/csharpgeneral/thread/74d03fe0-0fa5-438d-80e0-cf54fa15af0e

void A()  
{  
  Popup parameter = new Popup();  
  buttonClose.Click += (sender, e) => { buttonClose_Click(sender, e, parameter); };  
}  

static void buttonClose_Click(object sender, EventArgs e, Popup parameter)     
{     
  MakeSomethingWithPopupParameter(parameter);  
}

In my case, I was using a context menu for a TreeView control, which ended up looking like this:

private void TreeViewCreateContextMenu(TreeNode node)
{
    ContextMenuStrip contextMenu = new ContextMenuStrip();

    // create the menu items
    ToolStripMenuItem newMenuItem = new ToolStripMenuItem();
    newMenuItem.Text = "New...";

    // add the menu items to the menu
    contextMenu.Items.AddRange(new ToolStripMenuItem[] { newMenuItem });

    // add its event handler using a lambda expression, passing
    //  the additional parameter "myData"
    string myData = "This is the extra parameter.";
    newMenuItem.Click += (sender, e) => { newMenuItem_Click(sender, e, myData); };

    // finally, set the node's context menu
    node.ContextMenuStrip = contextMenu;
}

// the custom event handler, with "extraData":
private void newMenuItem_Click(object sender, EventArgs e, string extraData)
{
    // do something with "extraData"
}

call an eventhandler with arguments

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