Pass extra parameters to an event handler?

Question

Let's say I want to pass some extra data when assigning an event handler. Consider the following code:

private void setup(string someData)
{
     Object.assignHandler(evHandler);
}

public void evHandler(Object sender)
{
    // need someData here!!!
}

How would I go about getting someData into my evHandler method?

Answer 1

private void setup(string someData)
{
     Object.assignHandler((sender) => evHandler(sender,someData));
}
public void evHandler(Object sender, string someData)
{
    // need someData here!!!
}

Answer 2

I had a hard time figuring out @spender's example above especially with: Object.assignHandler((sender) => evHandler(sender,someData)); because there's no such thing as Object.assignHandler in the literal sense. So I did a little more Googling and found this example. The answer by Peter Duniho was the one that clicked in my head (this is not my work):

snip

The usual approach is to use an anonymous method with an event handler that has your modified signature. For example:

void Onbutton_click(object sender, EventArgs e, int i) { ... }

button.Click += delegate(object sender, EventArgs e) 
{ Onbutton_click(sender, e, 172); };

Of course, you don't have to pass in 172, or even make the third parameter an int. :)

/snip

Using that example I was able to pass in two custom ComboBoxItem objects to a Timer.Elapsed event using lambda notation:

simulatorTimer.Elapsed +=
(sender, e) => onTimedEvent(sender, e,
(ComboBoxItem) cbPressureSetting.SelectedItem,
(ComboBoxItem) cbTemperatureSetting.SelectedItem);

and then into it's handler:

static void onTimedEvent(object sender, EventArgs e, ComboBoxItem pressure, ComboBoxItem temperature)
    {
        Console.WriteLine("Requested pressure: {0} PSIA\nRequested temperature: {1}° C", pressure, temperature);
    }

This isn't any new code from the examples above, but it does demonstrate how to interpret them. Hopefully someone like me finds it instructive & useful so they don't spend hours trying to understand the concept like I did.

This code works in my project (except for a non-thread-safe exception with the ComboBoxItem objects that I don't believe changes how the example works). I'm figuring that out now.

Answer 3

Captured variables:

private void setup(string someData)
{
    Object.assignHandler((sender,args) => {
        evHandler(sender, someData);
    });
}

public void evHandler(Object sender, string someData)
{
    // use someData here
}

Or (C# 2.0 alternative):

    Object.assignHandler((EventHandler)delegate(object sender,EventArgs args) {
        evHandler(sender, someData);
    });

Answer 4

you can try doing this:

string yourObject;

theClassWithTheEvent.myEvent += (sender, model) =>
{
 yourObject = "somthing";
}

Answer 5

My question that was similar was marked a duplicate so thought I'd add an answer here since it won't let me on my question.

class Program
    {
        delegate void ComponentEventHandler(params dynamic[] args);

        event ComponentEventHandler onTest;

        static void Main(string[] args)
        {  
            Program prg = new Program();

            // can be bound to event and called that way
            prg.onTest += prg.Test;
            prg.onTest.Invoke("What", 5, 12.0);

            Console.ReadKey();
        }

        public void Test(params dynamic[] values)
        {
            // assign our params to variables
            string name = values[0];
            int age = values[1];
            double value = values[2];

            Console.WriteLine(name);
            Console.WriteLine(age);
            Console.WriteLine(value);
        }
    }

Answer 6

You could create a custom object having additional properties based on Object:

class CustomObject : Object
{
    public string SomeData;
}

private void setup(string someData)
{
    CustomObject customObject = new CustomObject { SomeData = someData };
    CustomObject.assignHandler(evHandler);
}

public void evHandler(Object sender)
{
    string someData = ((CustomObject)sender).SomeData;
}

If the data should not be changed anymore after initialization, you could also add a custom constructor, for example.

Answer 7

Here is my one-line solution that pass extra parameters to a timer handler.

private void OnFailed(uint errorCode, string message)
{
    ThreadPoolTimer.CreateTimer((timer) => {
    UI.ErrorMessage = string.Format("Error: 0x{0:X} {1}", errorCode, message);
    }, System.TimeSpan.FromMilliseconds(100));
}

Answer 8

Well, the simplest method id to make someData a member variable like so:

public class MyClass
{
    private string _eventData;

    private void setup(string someData) 
    {
       _eventData = someData;
       Object.assignHandler(evHandler);
    }

    public void evHandler()
    {
        // do something with _eventData here
    }
}

I'm not sure that's the best way to do it, but it really depends on the event type, the object, etc.

Answer 9

This solution offers a way to pass extra parameters to an event handler while still allowing to unsubscibe:

Within the Subscribe() function of my example I create an Action that invokes a lambda function that supplies my event handler with the event args and my extra parameter. I then store this Action in a dictionary. When I want to unsubscribe, I can use the stored Actions to do so.

This works, I read the length of listeners before and after unsubscribing and it did decrease - you can unsubscribe again without problems.

    public class Player
    {
        public Action<JumpInfo> OnJump;
    }

    public class PlayerJumpListener
    {
        public List<Player> MyPlayerList;
        private Dictionary<Player, Action<JumpInfo>> _jumpActionsByPlayer = new Dictionary<Player, Action<JumpInfo>>();

        private void Subscribe()
        {
            foreach (Player player in MyPlayerList)
            {
                Action<JumpInfo> playerJumpAction = (jumpInfo) => HandlePlayerJump(jumpInfo, player);
                player.OnJump += playerJumpAction;

                _jumpActionsByPlayer.Add(player, playerJumpAction);
            }
        }

        private void Unsubscibe()
        {
            foreach (KeyValuePair<Player, Action<JumpInfo>> kvp in _jumpActionsByPlayer)
            {
                kvp.Key.OnJump -= kvp.Value;
            }
        }

        private void HandlePlayerJump(JumpInfo jumpInfo, Player player)
        {
            // player jumped
        }
    }

Answer 10

I scoured the internet before a coworker kindly helped me, and boy I felt dumb. Brackets is the solution for the EventHandler.

Ex.

event EventHandler<(int, bool)> EventName;

and then pick it up with:

private void Delegate_EventName(object sender, (int, bool) e)

you can then access the info:

var temp = e.Item1;<br>
var temp2 = e.Item2;<br>

or you can add names as you would expect for parameters and call them via e:

private void Delegate_EventName(object sender, (int num, bool val) e)

you can then access the info:

var temp = e.num;
var temp2 = e.val;