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On leaving a control, how can I give that control focus again?

I've got TextBoxes in a C# form. The user enters data, and then when they leave the control (almost always by hitting Tab), I check the data to make sure it's valid. If it is invalid, I want to highlight their text so they can immediately fix it rather than having to click it.
Right now, on Control.Leave, I validate their entry. This works just fine. However, since they hit Tab, right after they dismiss the error message, it goes on to the next object, even though I've got ((TextBox)sender).Focus();
How can I have the above line fire after the form Tabs to the next control.

You may want to look into Control.CausesValidation property
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.causesvalidation(v=vs.110).aspx
You can validate the control prior to the user leaving focus rather than waiting on Focus moving itself.
And here's MSDN documentation for Control.Validating event, does a good job at laying out the sequence of events when gaining / losing focus of a Control.
http://msdn.microsoft.com/en-us/library/system.windows.forms.control.validating(v=vs.110).aspx
Notice how Control.Validating and Control.Validated are launched prior to Control.LostFocus. You can perform your validation step prior to allowing the user to lose focus of your Textbox.
There's also a pretty good previous answer on stackoverflow.com which outlines how to do this: C# Validating input for textbox on winforms

If you handle the Control.Validating event, setting e.Cancel to true will stop the change of focus from occurring.
Note that this method will also stop buttons from working, so you may need to set Control.CausesValidation to false on certain buttons.
You will also need the following snippet on the main form to allow the close button to work:
protected override void OnFormClosing(FormClosingEventArgs e) {
e.Cancel = false;
base.OnFormClosing(e);
}

Try using the LostFocus event on the TextBox to Focus it again

Exiting application on X button of form pressed

I have a modal form (login) which I want that on closing it using "X" button, force application to exit.
I tried this:
private void Login_Deactivate(object sender, EventArgs e)
{
Application.Exit();
}
this cause my application to exit on pressing login button (closing). but I want exiting only on "X" button of window.
How can I do it?

Get rid of the Deactivate event, you don't need it.
The issue here is that you cannot tell that just the "X" button was pressed. What you really do is track all the other conditions that cause form close, and react when it's not one of those.
You should leverage the DialogResult enum, and allow the calling application to react, instead of trying to do it all in the Login form.
In your Login form constructor, set the property to None:
public Login()
{
DialogResult = DialogResult.None;
}
Then, in your button handler for "OK", set the property: DialogResult = DialogResult.OK.
If you have a specific Cancel event, you can set DialogResult = DialogResult.Cancel.
Ultimately, you could have a FormClosing event in the Login form, in which you can check this value and react. But more importantly, your calling application can check the result from the login form and react there as well.

I think you should use this.Parent.Application.Exit(), it kills whole application.

Instead of Deactivate event, subscribe to the "FormClosing" event of the "Login" form. Write the same code which you have written.
Keep the Deactivate event only if it is required to close the Application when you click outside login form.

Windows form has focus but doesn't process keyboard events

I have a strange problem:
I have a Form that I open using ShowDialog(). The form is populated with some buttons and comboboxes. One of the comboboxes is set as the ActiveControl of the Form and the Form has focus.
What I want to accomplish is that the user can enter its username immediately after the Form opens (without the need to select the combobox first). However, if I press the keyboard, nothing happens. However, when I first click on the form with the mouse, and then enter something using the keyboard, it works. I already tried a lot of things like calling Select() and Focus() on the form. I even tried to simulate a mouseclick event (OnMouseClick) on the Form without any luck.
Someone has an idea would could be the problem here?
many thanks
Chris

Try BringToFront()
var f = new Form1();
f.Show();
f.BringToFront();
Then just use Select on that control
private void Form1_Load(object sender, EventArgs e)
{
comboBox1.Select();
}

Assuming you're running ShowDialog() from another form it might be worth changing it to: ShowDialog(this) so the new form has the correct parent and the correct blocking behaviour. Without the 'this' I've seen forms open up behind other forms and other strange behaviour, including focus problems.
Just a thought.

Have you tried calling myComboBox.Focus();
Just because the form has focus doesn't necessarily mean any control within the form has focus. Also try checking the onKey events of both the form and the individual controls. This normally helps me diagnose what exactly has focus. If the form itself and non on its controls are getting any onKey events then try using form.Activate();

Showing a form via Form.Show is not given focus?

I two forms, Form1, and a UserControl which hosts Form2. Within that UserControl on Form1 I call Form2.Show();. I have also tried Form2.Show(this);. Either way, the focus is not given to the form. I have to click once within the new form to give it focus, and then I can click items within that form.
I figured that control is passing back to my main control/form, and thus the focus is getting lost. So I am waiting until Form2 is closed via:
while (form2.Visible == true)
{
System.Threading.Thread.Sleep(100);
Application.DoEvents();
}
This seems to work. However after I close the form, now the reverse holds true. Form1 is not given focus (even if I call this.Focus()) until I click once within the main form window.
Any ideas how to handle this properly. I want to show a child form (modeless) and immediatley be able to click on it, and when that form is closed, immediately be able to take action back on the parent form.

You should probably use .ShowDialog(), this can also be extended to give your response on if the user performed Form2's operation properly or aborted early.
This does make the form locked in focus up front and will halt your code execution on the first form till that form is closed.

use this.Activate(); in place of this.Focus();

Not sure I follow completely, but from your UC try opening Form2 like this:
form2.Show(Parent);
This should specify the UC's Parent form as form2's owner.

This came from the fact that I was overriding WndProc in order to display the form. When I received the CBN_DROPDOWN message, I would display the form. I fixed this by instead Invoke'ing the method that shows the form and it fixed it.
case CBN_DROPDOWN:
Invoke(new MethodInvoker(Show_DropDown));
return;

Interacting with a form without it activating

I'm trying to implement code-completion popup window in my project. The window is derived from Form. It contains two controls: custom list derived from UserControl (it shows completion possibilities with icons) and a VScrollBar.
When the popup appears, it doesn't steal focus from the editor (form's ShowWithoutActivation is overriden to return true) and the editor sends certain keystrokes to the popup so the user can interact with it using keyboard. So far it works like a charm.
The problem is, I want to allow the user to use mouse as well. But, when the user clicks into the popup window, its form activates and steals focus from the editor. I can react to this by giving the focus back to the editor, I have even set up a Timer to do this regularly, but apart from being a poor solution, the title bar of the editor always flickers when this happens (when the popup is clicked).
Is there any way to interact with the popup form (using mouse) that doesn't make the form activate?
The ShowWithoutActivation's documentation reads: "If your non-activated window needs to use UI controls, you should consider using the ToolStrip controls, such as ToolStripDropDown. These controls are windowless, and will not cause a window to activate when they are selected." This seems exactly like the thing I need, but I want to use a custom control and a scroll bar.
The same problem would be with a tooltip that shows these two arrows to switch method overloads (known from VS) - the whole form would use no controls at all (only render the text and the arrows), but when clicked, it should not activate. The problem could be summarized up to "How to create a form that would never activate, but allow the user to interact with certail controls inside?".
Thanks.

Just override the onFocus event...
public partial class myListBox:ListBox
{
protected override void OnGotFocus(EventArgs e)
{
}
}

The issue is that you're using a Form for this rather than building some custom control that doesn't run in its' own UI thread like a Form does.
The flashing and highlighting is handled by windows whenever a Form activates/focuses. The only thing I cay think of is to make your Form borderless and create/draw/handle your own title bar that doesn't flash when focused.

OK, I may have found a solution. The key seems to be WM_MOUSEACTIVATE message, which the popup form must intercept and respond with MA_NOACTIVATE. But there's a catch - the control derived from UserControl still grabs focus when clicked (the scrollbar luckily doesn't anymore). The problem seems to be in the UserControl.OnMouseDown method, which internally puts focus on the control. There are some ways to fix this:
derive the control from Control instead of UserControl
override the OnMouseDown method and not call base.OnMouseDown there
make the control's CanFocus property return false, but this seems not possible, because that means to make the control either not visible or not enabled, which is both undesirable
The last case when the popup form steals focus seems to be when its resizing (using mouse) ends. But it is safe here to call Owner.Activate() as a result to Activated event...