Is there any method or Is it possible to write a method to execute a set of instructions when a (Windows Form) checkbox is unchecked (but not when checked)? I will clarify this providing my sample code.
Existing Sample Code:
private void ChkBox_CheckedChanged(object sender, EventArgs e)
{
if (ChkBox.Checked == false)
{
MessageBox.Show("unchecked");
}
}
Q: Could that if condition be avoided by writing a method for this which gets executed only when checkbox is unchecked.
Illustration:
Something like this:
private void ChkBox_Unchecked(object sender, EventArgs e)
{
MessageBox.Show("unchecked");
}
additional comments: I don't have anything to execute for event 'checkbox is checked' .. So just thinking if I can avoid checkchanged event by replacing it with unchecked type of event.
If you really need that strange events, then create custom check box by inheriting from CheckBox. Override it's OnCheckedChanged event, and add your Unchecked event:
public class StrageCheckBox : CheckBox
{
public event EventHandler Unchecked;
protected override void OnCheckedChanged(EventArgs e)
{
base.OnCheckedChanged(e);
if (!Checked)
OnUnchecked(e);
}
protected virtual void OnUnchecked(EventArgs e)
{
if (Unchecked != null)
Unchecked(this, e);
}
}
Use StrangeCheckBoxes instead of default check boxes. Subscribe to Unchecked event. Voila, you have that strange behavior.
Remark Actually I would not suggest usage of custom control or if/else in all scenarios. If you have many checkboxes with this kind of behavior, then custom control is better than many duplicated if/else in each handler. If you have several controls, or single handler for all checkboxes, then I'd go with if/else approach.
You can inherit the CheckBox and override OnCheckedChanged
class ExtendedCheckBox : CheckBox
{
public event EventHandler Unchecked;
protected override void OnCheckedChanged(EventArgs e)
{
base.OnCheckedChanged(e);
if (Checked == false)
OnUnchecked(e);
}
}
No, the only event exposed is one to detect when a change to the state occurred, then you must check the state to determine how it changed.
You could, if so inclined, create your own control which essentially wraps this one and exposes such state-distinguishing events, but this seems entirely pointless.
No, sorry. That's the only event you can trigger for a state change. Your code looks fine.
Any reason you don't like it?
As far as I know the event is fired on all changes, you could write your own control which differentiates the value selected and raises separate events but not sure it is really worthwhile.
I don't think you can do something like this, but if you have many instructions to execute if the checkbox was unchecked and you don't want to encapsulate them in an if statement you could check for checked at the beginning and return:
private void ChkBox_CheckedChanged(object sender, EventArgs e)
{
if (ChkBox.Checked) return;
MessageBox.Show("unchecked");
}
This will be a simple straight forward solution although the best suggested way will be to inherit from the CheckBox and modify the events. But, a relatively simple way is as follows:
Reduce the CheckBox width so that the CheckBox Label is no longer visible. Only the box should be visible. Then, include another label beside the box which should be mutually exclusive from the box.
Now, edit the CheckBox Clicked event. Use some sort of a flag to identify the state of the checkbox. Voila!
Related
I have an ASP DropDownList control with the AutoPostBack property set to true. When the user changes the selection, the form posts back as expected. I want to know how to determine, in the code-behind, whether the page posted back for this specific reason.
I know that I can define an event handler like this...
protected void MyDropDownList_SelectedIndexChanged(object sender, EventArgs e) {
// Run some code if the selection for the "MyDropDownList" control was changed
}
...but what I want to know is how to check whether the form posted back because the selected index was changed outside the event handler.
Specifically, in the Page_Load() method, I have an if (IsPostback) {} section, and I want this section not to execute if the postback was caused by changing the selection in the DropDownList. So, in pseudocode, I want something like:
if (IsPostback && (! <DropDownList's selection was changed, causing an autopostback>)) {
I tried defining a global boolean variable and setting it to true in an event handler, then checking it in Page_Load(), like this:
public partial class MyWebApp : System.Web.UI.Page {
[...]
static bool selectedIndexChanged = false;
[...]
protected void DomainDropDownList_SelectedIndexChanged(object sender, EventArgs e) {
selectedIndexChanged = true; // Set this flag to true if selected index was changed
}
[...]
protected void Page_Load(object sender, EventArgs e) {
[...]
if (IsPostBack && selectedIndexChanged == false) {
[...]
}
[...]
This didn't work for a reason I presume experienced ASP.NET developers will easily spot: the event handler executes after Page_Load(), regardless of the order of the code.
I also tried to see if the control's selectedIndexChanged event can be used as a boolean condition to determine if the event triggered, like this
if (IsPostBack && !MyDropDownList.SelectedIndexChanged) {
but Visual Studio gives me the following error:
The event 'System.Web.UI.WebControls.ListControl.SelectedIndexChanged' can only appear on the left hand side of += or -="
A search on the error message led to this answer, but that doesn't seem to help because it relies on the event handler, which executes after Page_Load().
In my particular use case, where there is only one DropDownList and only one other way to submit the form (the submit button), it would be equally effective to check whether the selected index was changed, whether an AutoPostBack was triggered, or whether the submit button was clicked, but I'd also like to know how to do this in a broader range of scenarios, for example if there are multiple AutoPostBack controls and/or multiple ways to submit the form other than AutoPostBack.
So, my question breaks down as follows (though some of these may be essentially the same question, depending on what the answer is):
Is there a way to determine in general whether an AutoPostBack was triggered, as opposed to the form posting back for any other reason, such as clicking a button?
Is there a way to determine if a specific control's AutoPostBack was triggered (i.e. if there are multiple controls with AutoPostBack true, can it be determined which control caused the AutoPostBack)?
Is it possible to check in the Page_Load() method or any other code that executes before the SelectedIndexChanged event handler whether a DropDownList's selected index was changed?
If there's a better way to achieve what I'm trying to accomplish in this particular case, I'm open to suggestions, but I'd still like to know the answer(s) to the above.
During Page_Load inspect Page.Request.Form["__EVENTTARGET"]. This will contain an identifier representing the control that caused the post back. From this you should be able to determine whether the post back was caused by the control you're interested in.
if (IsPostBack && Request.Form["__EVENTTARGET"] != "<control ID>") {
...
}
Is it possible to check in the Page_Load() method or any other code that executes before the SelectedIndexChanged event handler whether a DropDownList's selected index was changed?
Not without resorting to a custom technique. The SelectedIndexChanged event fires too late during the page event lifecycle to be useful to your scenario. One option is to store the DropDownList.SelectedIndex into the Page.ViewState collection during Page.OnPreRender and then comparing this value to the new DropDownList.SelectedIndex on the post back during Page_Load.
I have a WinForm which contains a multitude of controls interdependent on each other for their visibility and content.
I have a pair of radio buttons, controlling a combobox's (ComboBoxA) enable/disable flag and content. The selection on this combobox controls the visibility of a checkbox. The checking of this checkbox controls another combobox's (ComboBoxB) visibility and content. Business requirements are quite complicated around these controls. As a result, I require the ability to fire of the events programmatically and through user action, doing different things in each case.
In the checkbox's case, I check it programmatically while loading data (if needed), which fires the CheckedChanged event which in turn does additional action controlling ComboBoxB. The code for this is pretty vanilla, nothing special, but my question is more theoretical than practical. Please keep reading.
Due to this requirement, I need a way to distinguish between programmatic checking and user action. I tried using the Click event and CheckedChanged event, setting a flag in the click event, signifying user action. Unfortunately, the CheckedChanged event fires before the Click event, dead-ending this trick.
Now, I tried using the MouseDown event to capture user action. But funnily enough, once the event fires, checkbox remains unchecked and the CheckedChanged event doesnt fire.
Now, I have managed to use a flag in the code to determine programmatic checking and use that to distinguish between the two, but I was curious as to why the MouseDown event didnt allow the checkbox to be checked. Any ideas? I searched online but either I didnt do a thorough job of it, or google is not returning the right results for me. I apologize if anybody is actually able to find a google result for this problem.
It's something else in your code, not the MouseDown event that's preventing the CheckChanged to be fired.
Here is how I know this:
I've added a checkbox and a button to an empty form, and added event handlers to Click on the button, and on the checkbox CheckedChanged, KeyDown and MouseDown events. I've also added to the form a string variable called LastEventRaised, and in the CheckedChanged I've simply shown a MessageBox:
string LastEventRaised = string.Empty;
private void checkBox1_CheckedChanged(object sender, EventArgs e)
{
MessageBox.Show("Checked changed " + LastEventRaised);
LastEventRaised = string.Empty;
}
private void checkBox1_KeyDown(object sender, KeyEventArgs e)
{
LastEventRaised = "KeyDown";
}
private void checkBox1_MouseDown(object sender, MouseEventArgs e)
{
LastEventRaised = "MouseDown";
}
private void button1_Click(object sender, EventArgs e)
{
LastEventRaised = "programmatically";
checkBox1.Checked = !checkBox1.Checked;
}
Each time the message box popped up I've got the correct message.
I need help on firing an event within C#
Basically I have a onclick event that fires when you click on a checkbox
void OnClick(object sender, RoutedEventArgs e)
{
...
}
I need help on firing an event within C#
Basically I have a onclick event that fires when you click on a checkbox
void OnClick(object sender, RoutedEventArgs e)
{
...
}
However, I need to fire this event once another event has been fired, so within this new event, is it possible I can fire the above one?
private void DataGridCell_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
switch(dataGrid.Name)
{
case "Customer"
//fire OnCLick Event
break;
}
}
I have tried something like
??? += new MouseEventHandler(OnClick);
But I am not sure if this will actually work.
Yes you can, but only if the event is in your own class. You can't even raise a base class' event. You have a put a method in the base class to raise the event, and then call that.
The code you put there is adding another event handler, not raising an event; you don't need to do that.
If it's a button, use btnDoSomething.PerformClickEvent (winforms)
If the handler is in your code, you can call it without raising the event (commenters assume that this is what you want to do but in reaslity there are many cases where you'd need more than this) btnDoSomething_Click(null, null) - null usually works because handler code rarely cares about the sender or arguments and if you don't reference them, you don't need them.
If you can use #4, you can also refactor as mentioned. Usually not needed. But usually so easy to do you it's worth doing for clarity anyway.
For objects that map from Windows widgets of anysort, check out the SendMessage and PostMessage API calls. Wayyyy beyond the scope of this answer, though. Doesn't apply to non-windows-backed objects (but your sample implies windows).
I have a CheckListBox in c# and I am trying to trigger an event whenever one of the checkstates in the box is changed. The event purpose is to change some RichTextBox.
I have this piece of code, but it triggers the event only when one of the check boxes is turning from checked to unchecked, for some reason.
I tried to figure out what is wrong with my code with no success.
Any help will be appreciated.
private void clbAllRooms_ItemCheck(object sender, ItemCheckEventArgs e)
{
//If the checkstate changed, update price
//It updates price only when the state turns from Checked to Uncheck
if (e.NewValue != e.CurrentValue)
Update_rtbPrice();
}
The trouble is no doubt located in your Update_rtbPrice() method. It would have to call the list box' GetItemChecked() method to do something meaningful and that's a problem when you make the method call from the event handler. The item check state doesn't change until after the event runs.
A workaround is to delay the call so it runs after the control's state is updated. Like this:
private void clbAllRooms_ItemCheck(object sender, ItemCheckEventArgs e) {
this.BeginInvoke(new MethodInvoker(() => Update_rtbPrice()));
}
How to raise the SelectedIndexChanged event of an asp.net List control in a codebehind using C#?
If you're asking how to manually fire the event so that it can run whatever logic is attached: don't.
Your event handlers should be slim. If you need to perform the same operation from multiple places, then extract that functionality into its own method and have the event handler invoke that. For example:
private void CountryListBox_SelectedIndexChanged(object sender, EventArgs e)
{
UpdateStates(ListBox1.SelectedItem.Text);
}
private void UpdateStates(string country)
{
StateListBox.DataSource = GetStates(country);
StateListBox.DataBind();
}
Now instead of trying to fire the SelectedIndexChanged event, you just invoke the method that this event handler refers to, i.e.
private void Page_Load(object sender, EventArgs e)
{
UpdateStates("USA");
}
Don't put complex logic in event handlers and try to raise those events from unexpected places. Instead, put the complex logic in its own method, so that you can perform the associated actions from elsewhere.
It is raised automatically.
Go in the Events section, lightening
bolt in properties window
alt text http://img704.imageshack.us/img704/6100/listbox.jpg
double click the place holder next to
event. This is what you will get.
protected void ListBox1_SelectedIndexChanged(object
sender, EventArgs e)
{
}
if you want to raise this event from another code block then, call
ListBox1_SelectedIndexChanged(sender,
e);
If what you want is more than just executing the code behaviour coded for the selected index (like listed in the previous answer), the short answer is there is no easy way. You can write a simple code that on prerender or render to explicitly define the control id variable in your rendered HTML and then use javascript to set the selected index. This will cause the postback that trigger the event. Alternatively you can register an ajax call back method and have the client calls that either when some event happened or by automatic timer.