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 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

You're on the right track. You will need to override the SnapLines property in your designr and do something like this:

Public Overrides ReadOnly Property SnapLines() As System.Collections.IList
    Get
        Dim snapLinesList As ArrayList = TryCast(MyBase.SnapLines, ArrayList)

        Dim offset As Integer
        Dim ctrl As MyControl = TryCast(Me.Control, MyControl)
        If ctrl IsNot Nothing AndAlso ctrl.TextBox1 IsNot Nothing Then
            offset = ctrl.TextBox1.Bottom - 5
        End If

        snapLinesList.Add(New SnapLine(SnapLineType.Baseline, offset, SnapLinePriority.Medium))

        Return snapLinesList

    End Get
End Property

In this example the usercontrol contains a textbox. The code adds a new snapline that represents the baseline for the textbox. The important thing is to calculate the offset correctly.

Answer 2

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.

Answer 3

As an update to the Miral's answer.. here are a few of the "missing steps", for someone new that's looking how to do this. :) The C# code above is almost 'drop-in' ready, with the exception of changing a few of the values to reference the UserControl that will be modified.

Possible References Needed:
System.Design (@robyaw)

Usings needed:

using System.Windows.Forms.Design;
using System.Windows.Forms.Design.Behavior;
using System.ComponentModel;
using System.ComponentModel.Design;

On your UserControl you need the following Attribute:

[Designer(typeof(MyCustomDesigner))]

Then you need a "designer" class that will have the SnapLines override:

private class MyCustomerDesigner : ControlDesigner {
  public override IList SnapLines {
    get {
     /* Code from above */
    IList snapLines = base.SnapLines;

    // *** This will need to be modified to match your user control
    MyControl control = Control as MyControl;
    if (control == null) { return snapLines; }

    // *** This will need to be modified to match the item in your user control
    // This is the control in your UC that you want SnapLines for the entire UC
    IDesigner designer = TypeDescriptor.CreateDesigner(
        control.textBoxValue, typeof(IDesigner));
    if (designer == null) { return snapLines; }

    // *** This will need to be modified to match the item in your user control
    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)
            {
                // *** This will need to be modified to match the item in your user control
                snapLines.Add(new SnapLine(SnapLineType.Baseline,
                    line.Offset + control.textBoxValue.Top,
                    line.Filter, line.Priority));
                break;
            }
        }
    }

    return snapLines;
}

    }
  }
}

Answer 4

VB.Net Version: Note: you have to change the txtDescription to the Textbox or another internal control name that you use. and ctlUserControl to your usercontrol name

<Designer(GetType(ctlUserControl.MyCustomDesigner))> _
Partial Public Class ctlUserControl
   '... 
   'Your Usercontrol class specific code
   '... 
    Class MyCustomDesigner
        Inherits ControlDesigner
        Public Overloads Overrides ReadOnly Property SnapLines() As IList
            Get
                ' Code from above 

                Dim lines As IList = MyBase.SnapLines

                ' *** This will need to be modified to match your user control
                Dim control__1 As ctlUserControl = TryCast(Me.Control, ctlUserControl)
                If control__1 Is Nothing Then Return lines

                ' *** This will need to be modified to match the item in your user control
                ' This is the control in your UC that you want SnapLines for the entire UC
                Dim designer As IDesigner = TypeDescriptor.CreateDesigner(control__1.txtDescription, GetType(IDesigner))
                If designer Is Nothing Then
                    Return lines
                End If

                ' *** This will need to be modified to match the item in your user control
                designer.Initialize(control__1.txtDescription)

                Using designer
                    Dim boxDesigner As ControlDesigner = TryCast(designer, ControlDesigner)
                    If boxDesigner Is Nothing Then
                        Return lines
                    End If

                    For Each line As SnapLine In boxDesigner.SnapLines
                        If line.SnapLineType = SnapLineType.Baseline Then
                            ' *** This will need to be modified to match the item in your user control
                            lines.Add(New SnapLine(SnapLineType.Baseline, line.Offset + control__1.txtDescription.Top, line.Filter, line.Priority))
                            Exit For
                        End If
                    Next
                End Using

                Return lines
            End Get
        End Property
    End Class

End Class

Answer 5

Re: @Matthew M.'s answer, the System.Windows.Forms.Design.Behavior namespace can be found in the System.Design assembly.

Answer 6

Thanks to all those for the help. This was a tough one to swallow. The thought having a private sub-class in every UserControl wasn't very palatable.

I came up with this base class to help out..

[Designer(typeof(UserControlSnapLineDesigner))]
public class UserControlBase : UserControl
{
    protected virtual Control SnapLineControl { get { return null; } }

    private class UserControlSnapLineDesigner : ControlDesigner
    {
        public override IList SnapLines
        {
            get
            {
                IList snapLines = base.SnapLines;

                Control targetControl = (this.Control as UserControlBase).SnapLineControl;

                if (targetControl == null)
                    return snapLines;

                using (ControlDesigner controlDesigner = TypeDescriptor.CreateDesigner(targetControl,
                    typeof(IDesigner)) as ControlDesigner)
                {
                    if (controlDesigner == null)
                        return snapLines;

                    controlDesigner.Initialize(targetControl);

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

Next, derive your UserControl from this base:

public partial class MyControl : UserControlBase
{
    protected override Control SnapLineControl
    {
        get
        {
            return txtTextBox;
        }
    }

    ...

}

Thanks again for posting this.