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Why does GotFocus event keep deleting itself from Designer?

I wanted to add a GotFocus event to a Windows Forms Textbox, so I used the method described in this question; it works, but after I run my application a couple of times the piece of code deletes itself and I don't know why.
This is the code that keeps deleting itself:
txtID.GotFocus += txtID_GotFocus;

It disappears because you don't use conventions that are used by WinForms designer when you add event handlers.
It doesn't matter whether you use GotFocus or Enter event. If you (in your Designer.cs) manually add event handler this way:
txtID.Enter += txtID_Enter;
then it would always disappear from designer next time you move control on designer surface.
You must add event handlers this way:
txtID.GotFocus += new System.EventHandler(txtID_Focus);
txtID.Enter += new System.EventHandler(txtID_Enter);
and nothing would disappear because it's the way designer expects code to be.

Surely it's another evidence about why you should not touch designer generated code and should pay attention to this warning: do not modify the contents of this method with the code editor.
As a workaround use Enter event instead (which is recommended). Also you can assign the handler in your Load event of form.
EDIT
The reason is correctly mentioned by nikita, it's because you didn't use designer conventions. For more information see his answer.

C#.NET mnemonics unexpected behaviour

I have few buttons in my windows application which has mnemonics.Now if i press multiple keys the events of the button clicks gets fired as i have used mnemonics.
But the behaviour is not as expected,because second button event handler is getting executed before the first button event handler has finished its execution,and also i have written my code in such a way that in the event handler of first button i am disabling my second button,still the second button event handler is getting executed.The problem here is due to mnemonics.PLease suggest me a better way to handle mnemonics as soon as possible.
Thanks in advance.

If your logic is tied to your buttons, change that. Let your buttons merely manipulations an object that implements your logic. You can then check in this object whether the requested action is allowed (i.e. whether the previous operation has finished.
Alternatively you can just disable buttons when running and re-enable then when.finished.

How to handle complete change of the Textbox in Windows Forms?

What event I should handle to react to the completed change of the TextBox (i.e. when the user is
finished editing the content of the TextBox)?

There are several methods that you can use: Leave event, or a manual "typing stopped" event.
The Leave method is the most straight forward way of doing it, although as the event name suggests, it only happens when they TextBox looses focus, not when the user stops typing.
The TypingStopped event is something you would need to create yourself, but the basic idea of it is a short duration timer (say 500ms, but you would need to test it), which you restart on every KeyDown event of the TextBox. The timer would fire its own event and disable itself if it ever hits the end of it's timeout.
Edit: Updated to Leave event as per Hans' recommendation.

The Leave event is generally a good one for processing user input (for validation for example) as they move on to another part of the form. Just make sure that the event fires if they go from the textbox to any other UI element on your form - you may need to force a focus on the new element.

Can we Fire the button CLick event on Windowload In WPf C#

i want to FIre the button Click event When My Window is Loaded..
How Can i Achieve it in Wpf?

Create a single function with the shared behavior in your window, then call that function from both your loaded handler and your click handler.

As per this blog post in WinForms this was really easy by just calling PerformClick(), but in WPF you can do it with Automation, however as a commenter mentioned it's really easy if you have access to the button to just use RaiseEvent.
someButton.RaiseEvent(new RoutedEventArgs(Button.ClickEvent));
But as previously answered, if you only have a single handler that needs to be notified, then simply call that handler method directly.

You could use Automation to do it aswell - I've seen this suggested some places as the more flexible/robust method to use, but it seems a bit heavy weight to me compared to just calling the method you already have directly.

in you page_loaded event handler method, make a call to the click event like this:
_buttonName_click(sender, new RoutedEventArgs())

Trgger this event on Button whic u wanted to click
button.performclick();

Raise button (or any control) click event manually. C#

Can anyone tell me how to raise click event of button control (or for that matter for any event).
Platform: .net 2.0/3.0/3.5
Language: c#
Domain: Windows Application, WinForms, etc.

You can use the Button.PerformClick method.

Maybe the solution is much more simple:
Maybe you don't really want your code "to click the button".
Do you just want to run the code which is behind the button from another place in the form?
If yes, put the code into a separate method (like "DoActionXXX") and call the method from the button and from everywhere else where you need it.

You can also look into Windows Accessibility or some UI automation framework that allows you to programmatically cause UI controls to respond to user gestures. If a control does not offer a way for its events to be programmatically triggered like PerformClick, you can derive from that control and expose a public method like PerformnXXX that when called internally invokes the event handlers subscribed to a particular event.

button click and to new my manually window
ex.. dital.cs open the may window

I dont think sending click events is the best way from a design point of you just make your event handler call another function. That way you always have access to that function.
so ..
void myEventHandler(ObjectwhoSentTheMessage O, Event e)
{
MyOtherFunction();
//consume the event after if you wish etc
}