Could Label get a focus?_问答_开发者_运维开发者技术经验分享

I have a one question on my university's test about C#. Could label get a focus? As I can see on MSDN site, all Controls can get a focus, but some of them aren't selectable. So it's seems开发者_StackOverflow中文版 to me that the right answer is "Label could get a focus, but couldn't be selected". Also Label has a Focus() method. Please, help me understand. Thanx.

Yes there is a Focus() method on Label and yes it is absolutely right it works; but behave differently. let me try to explain

A Label can be associated with some one input control, for instance a label for a user name text field, so there is concept of Associated Control with the label. AssociatedControlID on msdn

So you can associate an input control with a label and whenever label is selected the control passed to the associated input control.

Example here click on Email or Password labels in login box and see what happened, similarly if you call focus method on the label the focus will passed to the associated control.

From the documentation:

A control can be selected and receive input focus if all the following are true: the Selectable value of ControlStyles is set to true, it is contained in another control, and all its parent controls are both visible and enabled.

Since a Label control is not selectable, it cannot receive input focus, even if it inherits a Focus() method from Control. Therefore, the answer is no.

It's easy to findo out if a control's ca get focus. Just read the

.CanFocus

property which is inherited from the Control class.

The Windows Forms controls in the following list are not selectable. Controls derived from these controls are also not selectable. (see MSDN documentation)

Panel
GroupBox
PictureBox
ProgressBar
Splitter
Label
LinkLabel (when there is no link present in the control)

Also:

The Focus method returns true if the control successfully received input focus. The control can have the input focus while not displaying any visual cues of having the focus. This behavior is primarily observed by the nonselectable controls listed below, or any controls derived from them.

A control can be selected and receive input focus if all the following are true: the Selectable value of ControlStyles is set to true, it is contained in another control, and all its parent controls are both visible and enabled.

If you need a Label-like control that you can focus, just use a TextBox and make it readonly. Set a few other properties (styles, not selectable etc.) and you're done.

You will see that there is a read only property called CanFocus on a label, if you have a look at this property while debugging you will see it is false.

Every control that inherits from Control has the focus method, but that does not mean that it can be focused.

Label does gets the focus but it escalates it to the input field specified in its "for" attribute. Like:

<label for="firstname">First Name</label><input type="text" name="firstname" />

In this scenario if you click on the label it will throw the focus to the input field "firstname" associated with it.

This is a year old, however I had a similar issue as the Op. In my case it was a user control that had a single label (docked at fill) on it (it has other functions behind the scenes - it is a calendar control and pops up a date picker - not the standard one - in either a panel (popunder) or a form (popup)).

The issue there was that UserControls are really intended as containers and resist focus (pushing it off to child controls) - as the label is the only child control, it stops the UserControl getting focus. Using readonly TextBox is a poor substitute as it lacks vertical alignment and must be multiline to size the height.

The reason I add the following as an answer here is because it IS possible (sorry guys who said here it is not) and I found this post and many like it that were little help when I looked. Anyway, the way to do it is to override the Label and set the SetStyle - also the OnPaint to draw a focus rectangle (I manually drew mine as DrawFocusRectangle didn't seem to do anything) - so as below:

internal class SelectableLabel: Label
{
    public SelectableLabel():base()
    {
        SetStyle(ControlStyles.Selectable, true);            
    }

    protected override void OnPaint(PaintEventArgs e)
    {
        base.OnPaint(e);

        //ControlPaint.DrawFocusRectangle(e.Graphics, ClientRectangle);
        if (!Focused) return;
        if (BorderStyle == BorderStyle.Fixed3D)
        {
            e.Graphics.DrawLines(Pens.CadetBlue, new[] { new Point(1, Height - 1), new Point(1, 1), new Point(Width - 1, 1) });
            e.Graphics.DrawLines(Pens.Aquamarine, new[] { new Point(2, Height - 1), new Point(Width - 1, Height - 1), new Point(Width - 1, 2) });
        }
        else
        {
            e.Graphics.DrawRectangle(Pens.Aquamarine, 0, 0, Width - 1 , Height - 1 );
        }
    }        
}

I am not concerning myself on whether it is accademically (purist view) right to do so, but that there are valid reasosn to allow an output control (like label) to sometimes gain focus.