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 create a designer (inherited from ControlDesigner) and override SnapLines to get access to the snaplines, but I’m wondering how to get the text baseline of a control that I have exposed by my custom user control.

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.