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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

How To Detect When Other Code Setting e.Handled = true?

Using C#, .NET 4, WPF.
I have a Telerik rich text control that is losing certain key events (tab, backspace, delete, and the arrow keys are specifics).
For debugging purposes I have added handlers for PreviewKeyDown, KeyDown, CommandExecuting, and DocumentContentChanged. The behavior presents both with and without the handlers present, in both DEBUG and RELEASE mode.
If I press a key other than those listed above I get the events in the order listed above. As an example, if I press the 'a' key I get PreviewKeyDown, KeyDown, CommandExecuting, and DocumentContentChanged.
If I press the right arrow key I get PreviewKeyDown and no other of the events fire.
My suspicion is that there is something trapping the KeyDown event at some point in the message chain before it gets to me and setting e.Handled = true.
Is there any tool available that would allow me to detect the KeyDown event and see in what code it's e.Handled is modified? I know I'm stretching here...
Thanks!
rjsjr

You could use Snoop. It can tell you, which Element set handled = true.
If you need to process these events, you can use EventManager.RegisterClassHandler().

DataGridViewTextBoxCell in edit mode steals keystrokes from Form even when using Form.KeyPreview

I have:
this.KeyPreview = true;
set in my Form, which allows me to typically receive KeyDown events even if a child element has focus. However, I have a DataGridView containing DataGridViewTextBoxCell's and when I click them, I enter "edit mode" for the cell and this somehow has such exclusive access to the keyboard that I can no longer receive these key presses. As long as anything in my entire Form has focus I want my keyboard functionality to actually work.
Does anyone know a way of getting these KeyDown events anyway?

Try handling the KeyUp event instead of KeyDown

Modify element property on key press

I need to hook up an event that shows a WPF Popup control when a TextBox has focus and a keyboard shortcut is clicked. For instance. When typing in the TextBox field, the user can press ALT+H for help, to get a popup dialog showing input help. Pressing ALT+H "outside" the TextBox should not open the popup.
Any ideas?

Looks like a job for an Attached Event.
From MSDN:
The concept of
an attached event enables you to add a handler for a particular event
to an arbitrary element rather than to an element that actually
defines or inherits the event. In this case, neither the object
potentially raising the event nor the destination handling instance
defines or otherwise "owns" the event.
You can find details here, on the MSDN

Use command binding:
ApplicationCommands.Help.InputGestures.Add(new KeyGesture(Key.H, ModifierKeys.Alt));
this.CommandBindings.Add(new CommandBinding(ApplicationCommands.Help, Help_Executed, Help_Enabled));
In function Help_Executed do some operations
In function Help_Enabled check if textbox selected, do e.CanExecute = true;
InputGestures assign ALT-H for help

textbox not losing focus

I have a Form which covers the entire screen. I have a textbox on it which is normally hidden but appears when a user clicks, drags mouse and then leaves. After that user can enter any value in the text box. Once entered, ideally user should be able to click outside of textbox and then the normal service should resume.
By normal service I mean that form should start getting all the events. What I have done so far is that on TextBox's KeyDown event; when Escape key is pressed, I have set the focus to the main form like this:
this.Focus(); //where this is mainform.
But this doesn't seem to work since Textbox still keeps receiving all the keys. I have a KeyDown event handler both for Form and Textbox and I have checked that all the KeyDown events pass on to the TextBox. I have a TextBox Leave event Handler as well which never gets called.
This TextBox is the only control on the form and the main form is used for drawing shapes (if that matters).
So, how can I make this TextBox lose focus when user clicks outside of it.

if it works like in VB, for what I remember, try to set the form property KeyPreview to false so all keys will be passed only to the focused control on the form.

If you set your form KeyPreview property to true, your form has first chance to handle any keystrokes that you make. If it is something you want to handle i.e. escape as in your comment above, handle it in your form's KeyDownEvent and mark it as handled so your textbox will not see it.
From above Msdn Page:
When this property is set to true, the form will receive all KeyPress, KeyDown, and KeyUp events. After the form's event handlers have completed processing the keystroke, the keystroke is then assigned to the control with focus.

I guess back in '11 this didn't work, but now
this.ActiveControl = null;
works fine. However if you intend to use Tab to cycle controls, focusing a label with suitable TabIndex is the way.