Baseline snaplines in custom Winforms controls

Question

I have a custom user control with a textbox on it and I\'d like to expose the baseline (of the text in the textbox) snapline outside of the custom control. I know that you

Answer 1

I just had a similar need, and I solved it like this:

 public override IList SnapLines
{
    get
    {
        IList snapLines = base.SnapLines;

        MyControl control = Control as MyControl;
        if (control == null) { return snapLines; }

        IDesigner designer = TypeDescriptor.CreateDesigner(
            control.textBoxValue, typeof(IDesigner));
        if (designer == null) { return snapLines; }
        designer.Initialize(control.textBoxValue);

        using (designer)
        {
            ControlDesigner boxDesigner = designer as ControlDesigner;
            if (boxDesigner == null) { return snapLines; }

            foreach (SnapLine line in boxDesigner.SnapLines)
            {
                if (line.SnapLineType == SnapLineType.Baseline)
                {
                    snapLines.Add(new SnapLine(SnapLineType.Baseline,
                        line.Offset + control.textBoxValue.Top,
                        line.Filter, line.Priority));
                    break;
                }
            }
        }

        return snapLines;
    }
}

This way it's actually creating a temporary sub-designer for the subcontrol in order to find out where the "real" baseline snapline is.

This seemed reasonably performant in testing, but if perf becomes a concern (and if the internal textbox doesn't move) then most of this code can be extracted to the Initialize method.

This also assumes that the textbox is a direct child of the UserControl. If there are other layout-affecting controls in the way then the offset calculation becomes a bit more complicated.