C# – in WinForms can control focus change “automatically”? if yes, can I somehow distinguish this from user triggered focus change

First of all, thanks for answers! 9 total answers. Thank you.

Bad news: all of the answers had some quirks or didn't work quite right (or at all). I've added a comment to each of your posts.

Good news: I've found a way to make it work. This solution is pretty straightforward and seems to work in all the scenarios (mousing down, selecting text, tabbing focus, etc.)

bool alreadyFocused;

...

textBox1.GotFocus += textBox1_GotFocus;
textBox1.MouseUp += textBox1_MouseUp;
textBox1.Leave += textBox1_Leave;

...

void textBox1_Leave(object sender, EventArgs e)
{
    alreadyFocused = false;
}


void textBox1_GotFocus(object sender, EventArgs e)
{
    // Select all text only if the mouse isn't down.
    // This makes tabbing to the textbox give focus.
    if (MouseButtons == MouseButtons.None)
    {
        this.textBox1.SelectAll();
        alreadyFocused = true;
    }
}

void textBox1_MouseUp(object sender, MouseEventArgs e)
{
    // Web browsers like Google Chrome select the text on mouse up.
    // They only do it if the textbox isn't already focused,
    // and if the user hasn't selected all text.
    if (!alreadyFocused && this.textBox1.SelectionLength == 0)
    {
        alreadyFocused = true;
        this.textBox1.SelectAll();
    }
}

As far as I can tell, this causes a textbox to behave exactly like a web browser's address bar.

Hopefully this helps the next guy who tries to solve this deceptively simple problem.

Thanks again, guys, for all your answers that helped lead me towards the correct path.

Seems like using the Enter and Leave events are the answer. GotFocus will only be sent to the specific control that gains focus, whereas the Enter event will also be sent to the parent (and ancestor) controls of the control that gets the GotFocus event.

from http://msdn.microsoft.com/en-us/library/system.windows.forms.control.leave.aspx

"The Enter and Leave events are hierarchical and will cascade up and down the parent chain until the appropriate control is reached. For example, assume you have a Form with two GroupBox controls, and each GroupBox control has one TextBox control. When the caret is moved from one TextBox to the other, the Leave event is raised for the TextBox and GroupBox, and the Enter event is raised for the other GroupBox and TextBox."