Public Class MainForm
    Inherits System.Windows.Forms.Form
    ''' <summary>
    ''' Main annotation viewer.
    ''' </summary>
    Private _viewer As Vintasoft.Imaging.Annotation.UI.AnnotationViewer

    ' ...

    Public Sub New()
        ' ...

        ' subscribe to the FocusedAnnotationViewChanged event of viewer

            ' ...
        AddHandler _viewer.FocusedAnnotationViewChanged, New System.EventHandler(Of Vintasoft.Imaging.Annotation.UI.AnnotationViewChangedEventArgs)(AddressOf _viewer_FocusedAnnotationViewChanged)
    End Sub

    ''' <summary>
    ''' Handler of the FocusedAnnotationViewChanged event.
    ''' </summary>
    Private Sub _viewer_FocusedAnnotationViewChanged(sender As Object, e As Vintasoft.Imaging.Annotation.UI.AnnotationViewChangedEventArgs)
        ' text transformer of old annotation
        Dim textTransformer As Vintasoft.Imaging.Annotation.UI.VisualTools.UserInteraction.RectangularTextAnnotationTransformer = Nothing
        If TypeOf e.OldValue Is Vintasoft.Imaging.Annotation.UI.TextAnnotationView Then
            textTransformer = DirectCast(e.OldValue, Vintasoft.Imaging.Annotation.UI.TextAnnotationView).TextTransformer
        ElseIf TypeOf e.OldValue Is Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView Then
            textTransformer = DirectCast(e.OldValue, Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView).TextTransformer
        End If

        If textTransformer IsNot Nothing Then
            ' unsubscribe from the TextBoxUpdated event of viewer
            RemoveHandler textTransformer.TextBoxTransformer.TextBoxUpdated, New System.EventHandler(Of Vintasoft.Imaging.UI.VisualTools.UserInteraction.TextObjectTextBoxTransformerEventArgs)(AddressOf textTransformer_TextBoxUpdated)
        End If

        ' text transformer of new annotation
        textTransformer = Nothing
        If TypeOf e.NewValue Is Vintasoft.Imaging.Annotation.UI.TextAnnotationView Then
            textTransformer = DirectCast(e.NewValue, Vintasoft.Imaging.Annotation.UI.TextAnnotationView).TextTransformer
        ElseIf TypeOf e.NewValue Is Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView Then
            textTransformer = DirectCast(e.NewValue, Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView).TextTransformer
        End If

        If textTransformer IsNot Nothing Then
            ' set new font of the text box
            textTransformer.TextBox.Font = New System.Drawing.Font("Arial", 20, System.Drawing.FontStyle.Bold)
            ' set text box location and size calculation mode to manual
            textTransformer.AutomaticallyCalculateTextBoxLocationAndSize = False
            ' subscribe to the TextBoxUpdated event of viewer
            AddHandler textTransformer.TextBoxTransformer.TextBoxUpdated, New System.EventHandler(Of Vintasoft.Imaging.UI.VisualTools.UserInteraction.TextObjectTextBoxTransformerEventArgs)(AddressOf textTransformer_TextBoxUpdated)
        End If
    End Sub

    ''' <summary>
    ''' Handler of the TextBoxUpdated event.
    ''' Location and size of the text box are setting to such values that text box matches
    ''' the visible part of the text annotation.
    ''' </summary>
    Private Sub textTransformer_TextBoxUpdated(sender As Object, e As Vintasoft.Imaging.UI.VisualTools.UserInteraction.TextObjectTextBoxTransformerEventArgs)
        ' get the text annotation
        Dim textAnnotation As Vintasoft.Imaging.Annotation.UI.VisualTools.UserInteraction.IRectangularTextAnnotation = DirectCast(e.TextObject, Vintasoft.Imaging.Annotation.UI.VisualTools.UserInteraction.IRectangularTextAnnotation)
        ' get bounding box of the annotation in the coordinate system of the viewer
        Dim annotationBoundingBox As System.Drawing.RectangleF = Vintasoft.Imaging.Utils.GraphicsUtils.TransformRect(textAnnotation.GetBoundingBox(), textAnnotation.GetPointTransform(_viewer, _viewer.Image))
        ' intersect with the visible area of the viewer
        Dim newBounds As System.Drawing.Rectangle = System.Drawing.Rectangle.Intersect(System.Drawing.Rectangle.Round(annotationBoundingBox), _viewer.ClientRectangle)
        ' set new bounds
        e.TextBox.Bounds = newBounds
    End Sub
End Class

public class MainForm : System.Windows.Forms.Form
{
    /// <summary>
    /// Main annotation viewer.
    /// </summary>
    Vintasoft.Imaging.Annotation.UI.AnnotationViewer _viewer;

    // ...

    public MainForm()
    {
        // ...

        // subscribe to the FocusedAnnotationViewChanged event of viewer
        _viewer.FocusedAnnotationViewChanged += 
            new System.EventHandler<Vintasoft.Imaging.Annotation.UI.AnnotationViewChangedEventArgs>(_viewer_FocusedAnnotationViewChanged);

        // ...
    }

    /// <summary>
    /// Handler of the FocusedAnnotationViewChanged event.
    /// </summary>
    void _viewer_FocusedAnnotationViewChanged(object sender, Vintasoft.Imaging.Annotation.UI.AnnotationViewChangedEventArgs e)
    {
        // text transformer of old annotation
        Vintasoft.Imaging.Annotation.UI.VisualTools.UserInteraction.RectangularTextAnnotationTransformer textTransformer = null;
        if (e.OldValue is Vintasoft.Imaging.Annotation.UI.TextAnnotationView)
        {
            textTransformer = ((Vintasoft.Imaging.Annotation.UI.TextAnnotationView)e.OldValue).TextTransformer;
        }
        else if (e.OldValue is Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView)
        {
            textTransformer = ((Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView)e.OldValue).TextTransformer;
        }

        if (textTransformer != null)
        {
            // unsubscribe from the TextBoxUpdated event of viewer
            textTransformer.TextBoxTransformer.TextBoxUpdated -= 
                new System.EventHandler<Vintasoft.Imaging.UI.VisualTools.UserInteraction.TextObjectTextBoxTransformerEventArgs>(textTransformer_TextBoxUpdated);
        }

        // text transformer of new annotation
        textTransformer = null;
        if (e.NewValue is Vintasoft.Imaging.Annotation.UI.TextAnnotationView)
        {
            textTransformer = ((Vintasoft.Imaging.Annotation.UI.TextAnnotationView)e.NewValue).TextTransformer;
        }
        else if (e.NewValue is Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView)
        {
            textTransformer = ((Vintasoft.Imaging.Annotation.UI.FreeTextAnnotationView)e.NewValue).TextTransformer;
        }

        if (textTransformer != null)
        {
            // set new font of the text box
            textTransformer.TextBox.Font = new System.Drawing.Font(
                "Arial", 20, System.Drawing.FontStyle.Bold);
            // set text box location and size calculation mode to manual
            textTransformer.AutomaticallyCalculateTextBoxLocationAndSize = false;
            // subscribe to the TextBoxUpdated event of viewer
            textTransformer.TextBoxTransformer.TextBoxUpdated += 
                new System.EventHandler<Vintasoft.Imaging.UI.VisualTools.UserInteraction.TextObjectTextBoxTransformerEventArgs>(textTransformer_TextBoxUpdated);
        }
    }

    /// <summary>
    /// Handler of the TextBoxUpdated event.
    /// Location and size of the text box are setting to such values that text box matches
    /// the visible part of the text annotation.
    /// </summary>
    void textTransformer_TextBoxUpdated(object sender, Vintasoft.Imaging.UI.VisualTools.UserInteraction.TextObjectTextBoxTransformerEventArgs e)
    {
        // get the text annotation
        Vintasoft.Imaging.Annotation.UI.VisualTools.UserInteraction.IRectangularTextAnnotation textAnnotation = 
            (Vintasoft.Imaging.Annotation.UI.VisualTools.UserInteraction.IRectangularTextAnnotation)e.TextObject;
        // get bounding box of the annotation in the coordinate system of the viewer
        System.Drawing.RectangleF annotationBoundingBox = Vintasoft.Imaging.Utils.GraphicsUtils.TransformRect(
            textAnnotation.GetBoundingBox(), textAnnotation.GetPointTransform(_viewer, _viewer.Image));
        // intersect with the visible area of the viewer
        System.Drawing.Rectangle newBounds = System.Drawing.Rectangle.Intersect(
            System.Drawing.Rectangle.Round(annotationBoundingBox), _viewer.ClientRectangle);
        // set new bounds
        e.TextBox.Bounds = newBounds;
    }
}