What I'm trying to do here is really simple but it's not working for me. I've looked up many examples of very similar or matching questions on stackoverflow and others and their solutions haven't worked for me.
Basically (in a very simplified form), I have a button with some text in it. I'd like it so when this button is clicked, a message box shows up displaying the text. However, I have 9 of these buttons (think numbers on a calculator), and I'd like to create a single method that handles all of these clicks, and outputs the correct text depending on the button.. which is why I need to pass the button as a parameter.
Here's the code I have for the method that handles it so far:
private void btn_Click(object sender, EventArgs e, Button b)
{
MessageBox.Show(b.Text);
}
The above small code snippet is the same solution that others have used and had working. However, for me this code doesn't compile and shows an error that says No overload for 'btn_Click' matches delegate 'System.EventHandler'.
When I double click on that error, it takes me to the Designer.cs page for the form, and this is the line of code that has the error:
this.btnN7.Click += new System.EventHandler(this.btnN7_Click);
I have no clue what the overload and delegate parts mean, sorry I'm pretty new to this. I was thinking that maybe overload has to do with constructors but even if that's correct, I'm unsure of what the next step would be.
I have System.Windows.Forms; included properly so the issue shouldn't be that the Button object wasn't recognized.
If you could provide any insight as to what I'm missing or doing wrong, that'd be very sweet! Please let me know if you need any additional information to continue.
And lastly, this is unrelated to my issue but it's a small question that's been irking me ever since I started using VS a week ago: Are the control parameters object sender and EventArgs e that are automatically created for controller events even necessary? Most of the examples I've looked up online omit them. I've just kept them in since they were created by default but I don't really know what kind of function they provide, and I've never had to use those parameters in my methods.
Thanks all! =)
You just need to use sender parameter.You can't subscribe an event handler if the method signature doesn't match with the EventHandler delegate.
private void btn_Click(object sender, EventArgs e)
{
var currentButton = sender as Button;
if(currentButton != null) MessageBox.Show(currentButton.Text);
}
sender will be assigned to an object that triggers the event.So for instance when your button2 is clicked, it will be assigned to button2.Ofcourse you need to attach this event handler to button2's Click event.I have also used the as operator to ensure that a button is triggered the event.In the future if you call this method manually like btn_Click(this, EventArgs.Empty) then the explicit cast will throw an InvalidCastException.Using as operator is always better to avoid this.If the type of the sender is different than Button then currentButton will be null.
which is why I need to pass the button as a parameter.
You don't really have to create that extra parameter. What you need is already there.
object sender is a source of the event, so just call
var btn = (Button) sender;
MessageBox.Show(btn.Text);
and you should be fine.
Are the control parameters object sender and EventArgs e that are automatically created for controller events even necessary?
So now you have a part of an answer for this question. The second part (EventArgs) are needed for passing additional information. Read more about event args on MSDN.
You have two options here:
If you want to access to the button that fired the event, you can do it trough sender
private void btn_Click(object sender, EventArgs e)
{
Button b = (Button)sender;
MessageBox.Show(b.Text);
}
Otherwise, access directly through the button name (in case you know it)
Related
What do sender and eventArgs mean/refer to? How can I make use of them (for the scenario below)?
Scenario:
I'm trying to build a custom control with a delete function, and I want to be able to delete the control that was clicked on a page that contains many of the same custom control.
The sender is the control that the action is for (say OnClick, it's the button).
The EventArgs are arguments that the implementor of this event may find useful. With OnClick it contains nothing good, but in some events, like say in a GridView 'SelectedIndexChanged', it will contain the new index, or some other useful data.
What Chris is saying is you can do this:
protected void someButton_Click (object sender, EventArgs ea)
{
Button someButton = sender as Button;
if(someButton != null)
{
someButton.Text = "I was clicked!";
}
}
sender refers to the object that invoked the event that fired the event handler. This is useful if you have many objects using the same event handler.
EventArgs is something of a dummy base class. In and of itself it's more or less useless, but if you derive from it, you can add whatever data you need to pass to your event handlers.
When you implement your own events, use an EventHandler or EventHandler<T> as their type. This guarantees that you'll have exactly these two parameters for all your events (which is a good thing).
Manually cast the sender to the type of your custom control, and then use it to delete or disable etc. Eg, something like this:
private void myCustomControl_Click(object sender, EventArgs e)
{
((MyCustomControl)sender).DoWhatever();
}
The 'sender' is just the object that was actioned (eg clicked).
The event args is subclassed for more complex controls, eg a treeview, so that you can know more details about the event, eg exactly where they clicked.
'sender' is called object which has some action perform on some
control
'event' its having some information about control which has
some behavoiur and identity perform
by some user.when action will
generate by occuring for event add
it keep within array is called event
agrs
FYI, sender and e are not specific to ASP.NET or to C#. See Events (C# Programming Guide) and Events in Visual Basic.
How can I register to event and do some actions at runtime?
For example when the user click on a button I want to register to OnMyEvent and run MyFunc that let's say initialize some textBox with the OnMyEvent args.
I'm using winforms and .NET 4.
EDIT
Maybe I was unclear... I need the ability to register to existing event and add a new method that will run when the event will fire. All at runtime.
EDIT2
i'll try to give an example...
lets say that i have a class named "A" that have many events OnDataRecived OnDataSend etc...
when the application running the user can choose form a combobox event name to register (i got the events list via reflection because they not constracts, they are generated from xml file) and which data to update when the choosed event is fired.
so for the example the user choose to register to the OnDataReceived and he choose to update property named DataStream. some code...
in run time upon user choosing:
A.OnDataReceived += (s,e) => MyRunTimeMethod(s,e);
private void MyRunTimeMethod(object sender, EventArgs e)
{
DataStream = e.Data.Value
}
You are asking how to create a method dynamically at runtime - once you have a reference to that method in a delegate, the question of how to register it to an event is trivial.
MSDN describes how to do this with MSIL instructions. I doubt that's what you're looking for, but it is an option.
The C# FAQ blog has a much more interesting solution using expression trees. I suppose this is the one you were referring to by originally tagging your post with expression-tree.
But I would reconsider using dynamic methods at all. How exactly is the user going to specify what action to perform on the event of his choice? I suspect that the options are limited enough that you can get by with something simpler:
protected void btnRegister_Click(object sender, EventArgs e) {
switch (cmbEvents.SelectedText) {
case "OnLoad":
MyControl.OnLoad += (s, e) => SomeSelectedControl.Text = SomeInputControl.Text;
break;
//... other cases
}
}
If you're using windows forms, double clicking a button will bring you to a created on_click event. If you bring up the properties window for the button, theres an events tab. Viewing this will show you which events are available for a control.
I found the best way to understand this, was to look at the code created when adding the events.
Update:
As noted, I completely missed the point with my answer. The syntax for subscribing to an event at runtime is the same way as it's done on form Initialize. So I don't get any terminology wrong, here's the link to the msdn documentation;
http://msdn.microsoft.com/en-us/library/ms366768.aspx
What you want to achieve, does not require you to "Register to event at run time".
If button1 is the button of interest here, simply use.
button1.Click += buton1_ClickHandler;
button1_ClickHandler should be defined in the same class as your button1. and it should have the signature of the RoutedEventHandler. So, it should be
private void button1_ClickHandler(object sender, RoutedEventArgs e)
{
//method code here
}
I've got a form with several controls (to make things simple, say it's a couple of textboxes), which I need to be updated by clicking on two buttons - a Forward button and a Backwards button.
Now, the general idea is that the information to be displayed is stored in some kind of array of objects, and is shown according to the value of some counter. That is, a user clicks on the Forward button -> the counter is incremented -> the corresponding array item is shown on the form. Same goes for the Backwards button.
So, the question is - should I define any specific event in this case? Or is it sufficient to use the standard
private void button1_Click(Object sender, EventArgs e) {}
event which is provided when double-clicking on a button control? I mean, what would be the right thing to do?
I think it's a pretty dumb question, still I appreciate your advice here, thanks in advance!
The click event is an ok place for that logic, however, it's a good practice to extract that forward/backward logic in a separate method (maybe you'll want to go forward by pressing the right arrow?) to something like this:
private void btnForward_Click(Object sender, EventArgs e)
{
GoForward();
}
private void GoForward()
{
// the forwarding code here
}
And, make a habit of naming controls as early as possible, because VS uses the control name to name the event handler, and button1_Click is not very descriptive :)
i have a form holding a button that, when clicked, needs to perform the actions within another class.
for example,
the user clicks a btn thats says BookNow.
the coding within the btn needs to point to the BookNow class which has information for booking.
can anyone help me with this issue please?
You need to create an instance of the BookNow class and call the function on it, from the button event handler.
public void button1_click(object sender, EventArgs e)
{
BookNow bn = new BookNow();
// Call methods/properties on bn
}
I would avoid calling the button with the same name of the class, as you will run into namespace and naming conflicts.
May be you are looking for command design pattern
http://www.dofactory.com/Patterns/PatternCommand.aspx#_self1
I have a function for which i am sharing a group of link buttons with.
the function signature is like:
protected void FunctionName(object sender, EventArgs e)
{
...
}
Now I have about 4-5 link buttons which i am using to call the same function but just filtering the stuff via command argument like:
<asp:LinkButton ID="lbAll" runat="server" Text="All"
CommandArgument="all" OnClick="FunctionName"></asp:LinkButton>
<asp:LinkButton ID="lbTop" runat="server" Text="Top"
CommandArgument="top" OnClick="FunctionName"></asp:LinkButton>
(...)
Now, I have a drop down box which needs to do the same thing essentially (on just two of the selected values), i just need to call this function and pass the "all" or "top" argument to the Function: "FunctionName"
this is in C#
I tried to call that function like
FunctionName(this, New EventArgs());
but I dont know how to pass the Argument?
Any ideas on this?
Thanks!
Pass the LinkButton with the correct CommandArgument instead of the this:
FunctionName(lbAll, EventArgs.Empty)
But you really should use the OnCommand event instead of OnClick. OnCommand has CommandEventArgs as second parameter. So you can get them with e.CommandArgument in the method. And call the method width:
FunctionName(this, new CommandEventArgs("CommandName", "CommandArgument"));
Editing for clarity
Your event handler for the click event will have two parameters, a sender and an event args object. The sender is the object that triggered the event (the linkbutton). In that event handler you can cast the sender to the right object type and access its properties.
((LinkButton)Sender).CommandArgument
Using this method you don't need to explicitly pass your argument, you just retrieve it from the sender.
Alternatively (and probably better) you can use the "OnComand" event handler. Looking up the property you are already using at http://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.linkbutton.commandargument.aspx you will see that this event handler receives a CommandEventArgs parameter that has a property that exposes the CommandParameter object. eg:
void LinkButton_Command(Object sender, CommandEventArgs e)
{
Label1.Text = "You chose: " + e.CommandName + " Item " + e.CommandArgument;
}
(from that MSDN page).
OK. I think I missed my point on the last answer having realised its a question of calling the event handler from somewhere in code that isn't that event...
What I would suggest is that you refactor your event handler to take out the functionality and put it into a separate method or two. eg in pseudocode:
protected void FunctionName(object sender, EventArgs e)
{
if (top)
DoTop();
else if (all)
DoAll();
}
private void DoTop()
{
//do stuff
}
private void DoAll()
{
//Do different stuff
}
This should make it really easy then to just call the bit of code that you need. Althoguh I'm not sure I suspect its considered bad practice to call an event handler just because you need some of its functionality. Its certainly looking like more work than it needs to be. :)
You could of course instead have a method that takes a parameter and then deals with it appropriately (effectively factoring out everything in FunctionName).
I think this answers your question better. If not I'm just going to go home. ;-)