Proposal: add SelectionStart and SelectionEnd to TrackBar

[hughbe]

You can set the TBS_ENABLESELRANGE on TrackBar and send the TBM_SETSELSTART and TBM_SETSELSTART to set the selection range
See docs

• https://docs.microsoft.com/en-us/windows/win32/controls/tbm-setselstart
• https://docs.microsoft.com/en-us/windows/win32/controls/tbm-setselend

Example:

We could add the following APIs to TrackBar:

public class TrackBar
{
    ...
    public bool ShowSelectionRange { get; set; }
    public int SelectionStart { get; set; }
    public int SelectionEnd { get; set; }
    ...
}

Discussion

• What should the default values of SelectionStart and SelectionEnd be if we haven't enabled selections? In my experiments I set them to 0 which worked nicely.
• In my prototype implementation, I set ShowSelectionRange to true if the user sets SelectionStart/SelectionEnd. One option could be to remove ShowSelectionRange and set SelectionStart/SelectionEnd to minus one or have some logic that sets the TrackBar style to TBS_ENABLESELRANGE if it is set to a custom value. However, this is negative as it leaves us with no way of removing the selection indicators once they've been created.
• Do we want SelectionStart and SelectionEnd or SelectionStart and SelectionLength or some sort of SelectionRange struct that encapsulates this? (e.g MonthCalendar.SelectionRange)

[weltkante]

Chosing -1 as sentinel value is bad because I think you can have a negative Minimum. Letting users chose a value centered around zero (e.g. between -100 and +100) is probably a common usecase of this control.

Having a separate boolean property is how this is usually done in WinForms so that would be my first choice here as well.

One question I'm not clear on is whether this is an input property (can the user edit the selection) or is it informative only (e.g. for presenting the current range of values within a multi-selection). If the user cannot edit this through user input I'm hesitant to name it "Selection" even if the native API names it this way.

[hughbe]

One question I'm not clear on is whether this is an input property (can the user edit the selection) or is it informative only (e.g. for presenting the current range of values within a multi-selection). If the user cannot edit this through user input I'm hesitant to name it "Selection" even if the native API names it this way.

From what I can see it is informative only. In my example gif I showed how it can be connected to the value of the thumb as an example use case. Open to any other suggestions for names (perhaps SelectionIndicatorStart?)

[weltkante]

I'm not too picky about naming, its just that Selection tends to be connected with user input. If there's desire to keep the name its ok as well, but I want it be discussed. At this point its probably best to throw a bunch of suggestions in, I'd be ok with SelectionIndicatorStart. Some alternatives could be dropping the Selection prefix (just IndicatorStart) or MarkerStart. "Indicator" and "Marker" are common terms in charting when annotating an axis and vaguely resemble the functionality of what we have here.

Do we want SelectionStart and SelectionEnd or SelectionStart and SelectionLength or some sort of SelectionRange struct that encapsulates this?

After thinking about this I tend to go for Start/End because this maps most closely to what the control is doing: the user is selecting a value and you put markers at two other values to mark a range. This allows easy binding of the properties against two values, going for Start/Length would more probably require coding for calculating the length.

Chosing a struct would probably not be very compatible with the WinForms binding system, but I can't tell for sure since there is little precedence. Maybe it would just work out firing INotifyPropertyChanged event when either property changes on the struct and the Binding engine will resolve the member access? Might worry about boxing though, I think binding works on objects. Strike that, structs are incompatible with the binding engine. Since this is not a user input you always will want to write it through the binding engine. You can't write struct members through the binding engine since they'll be a boxed copy and not affect the struct owned by the control.

[RussKie]

@hughbe could you please share your test app (e.g. publish it to GitHub under your account)? It would be great to have some hands on experience to make better informed decisions.

I'm leaning towards defining a new class similar to SelectionRange. This way we can provide a better the Designer support. And it would be consistent with the existing API.
It is unfortunate SelectionRange name is already taken. Though we can probably do a nested class within the TrackBar class.
Another control I can think of that exposes a selection configuration is LinkLabel.LinkArea. But it is not the same functionality as TrackBar though.

• What should the default values of SelectionStart and SelectionEnd be if we haven't enabled selections? In my experiments I set them to 0 which worked nicely.

These values must also be within the range.

• In my prototype implementation, I set ShowSelectionRange to true if the user sets SelectionStart/SelectionEnd.

What do you think of ShowSelection? This would compliment ShowThumb too...

[RussKie]

The team had some preliminary discussions about enhancing the control.
Could you spare few minutes and provide answers to the following questions so we can take it to the next level:

• Will VS Designer need to support the feature? If yes, describe how you expect it to funсtion.
• What impact will it have on accessibility?
• Will this feature need to be localized or be localizable?

With the regards to this specific feature the first question is where I expect most questions.

[terrajobst]

Video

• We should probably follow TextBoxBase and use SelectionStart and SelectionLength, because that avoids ordering issues and the need for sentinel values. @RussKie, it would be good to check what the throw behavior is for TextBoxBase.
• We considered making it method like SetSelection(int start, int length) but that would mean you can't set the selection using the designer, which feels odd

namespace System.Windows.Forms
{
    public partial class TrackBar
    {
        public bool ShowSelectionRange { get; set; }
        public int SelectionStart { get; set; }
        public int SelectionLength { get; set; }
    }
}

@hughbe What the other properties you show in your UI? Like ShowThumb and ThumbWidth?

[RussKie]

Few questions:

• Throw if SelectionLength < 0
• Clamp SelectionStart to either Minimum or Maximum if outside the boundaries
• Clamp if SelectionStart + SelectionLength > Maximum to Maximum

[elachlan]

@conorgee This is another trackbar improvement if you want to give it a go?