RichTextBox CreateCaret Width Measurement

Question

I'm extending the richtextbox by creating a caret for the insert and overwrite modes. The issue is that the width measured is too wide. This is most noticeable on larger font sizes.

The height returned from SelectionFont.Height is correct. Is there no similiar Width property?

How can I measure the width of the specified character ch accurately? Can anyone provide an example?

Below is the code converted to C#, and below that in VB.NET:

using System;

public class RichTextBoxCaret : RichTextBox
{
    [DllImport("user32.dll", CharSet=CharSet.Auto)]
    private extern static void CreateCaret(IntPtr hWnd, IntPtr hBitmap, int nWidth, int nHeight);

    [DllImport("user32.dll", CharSet=CharSet.Auto)]
    private extern static void ShowCaret(IntPtr hWnd);

    private bool mInsertKeyState = true;

    protected override void OnKeyDown(System.Windows.Forms.KeyEventArgs e)
    {
        base.OnKeyDown(e);

        if (e.KeyCode == Keys.Insert)
        {
            mInsertKeyState = !mInsertKeyState;
        }

        this.DrawCaret();
    }

    protected override void OnMouseUp(System.Windows.Forms.MouseEventArgs mevent)
    {
        base.OnMouseUp(mevent);

        this.DrawCaret();
    }

    public void DrawCaret()
    {
        Size sz = new Size(0, 0);

        if (this.SelectionFont != null)
        {
            if (!mInsertKeyState && this.SelectionStart < this.TextLength)
            {
                using (Graphics g = this.CreateGraphics)
                {
                    using (Font f = new Font(this.SelectionFont.FontFamily, this.SelectionFont.Size, this.SelectionFont.Style, GraphicsUnit.Pixel, Convert.ToByte(0), false))
                    {
                        char ch = this.Text[this.SelectionStart];
                        sz.Width = Convert.ToInt32(g.MeasureString(ch, f).Width * this.ZoomFactor);
                    }
                }
            }

            sz.Height = Convert.ToInt32(this.SelectionFont.Height * this.ZoomFactor);
        }

        if (!sz.IsEmpty)
        {
            CreateCaret(this.Handle, IntPtr.Zero, sz.Width, sz.Height);
            ShowCaret(this.Handle);
        }
    }
}

And for VB.NET:

Public Class RichTextBoxCaret
    Inherits RichTextBox

    <DllImport("user32.dll", CharSet:=CharSet.Auto)> _
    Private Shared Sub CreateCaret(ByVal hWnd As IntPtr, ByVal hBitmap As IntPtr, ByVal nWidth As Integer, ByVal nHeight As Integer)
    End Sub

    <DllImport("user32.dll", CharSet:=CharSet.Auto)> _
    Private Shared Sub ShowCaret(ByVal hWnd As IntPtr)
    End Sub

    Private mInsertKeyState As Boolean = True

    Protected Overrides Sub OnKeyDown(ByVal e As System.Windows.Forms.KeyEventArgs)
        MyBase.OnKeyDown(e)

        If e.KeyCode = Keys.Insert Then
            mInsertKeyState = Not mInsertKeyState
        End If

        Me.DrawCaret()
    End Sub

    Protected Overrides Sub OnMouseUp(ByVal mevent As System.Windows.Forms.MouseEventArgs)
        MyBase.OnMouseUp(mevent)

        Me.DrawCaret()
    End Sub

    Public Sub DrawCaret()
        Dim sz As New Size(0, 0)

        If Me.SelectionFont IsNot Nothing Then
            If Not mInsertKeyState AndAlso Me.SelectionStart < Me.TextLength Then
                Using g As Graphics = Me.CreateGraphics
                    Using f As New Font(Me.SelectionFont.FontFamily, Me.SelectionFont.Size, Me.SelectionFont.Style, GraphicsUnit.Pixel, CByte(0), False)
                        Dim ch As Char = Me.Text.Chars(Me.SelectionStart)
                        sz.Width = CInt(g.MeasureString(ch, f).Width * Me.ZoomFactor)
                    End Using
                End Using
            End If

            sz.Height = CInt(Me.SelectionFont.Height * Me.ZoomFactor)
        End If

        If Not sz.IsEmpty Then
            CreateCaret(Me.Handle, IntPtr.Zero, sz.Width, sz.Height)
            ShowCaret(Me.Handle)
        End If
    End Sub
End Class

Answer 1

Measuring the width was resolved by using GetPositionFromCharIndex.

The final implementation:

<DllImport("user32.dll", CharSet:=CharSet.Auto)> _
Private Shared Sub CreateCaret(ByVal hWnd As IntPtr, ByVal hBitmap As IntPtr, ByVal nWidth As Integer, ByVal nHeight As Integer)
End Sub

<DllImport("user32.dll", CharSet:=CharSet.Auto)> _
Private Shared Sub ShowCaret(ByVal hWnd As IntPtr)
End Sub

Private mInsertKeyState As Boolean = True

Protected Overrides Sub OnKeyDown(ByVal e As System.Windows.Forms.KeyEventArgs)
    MyBase.OnKeyDown(e)

    If e.KeyCode = Keys.Insert Then
        mInsertKeyState = Not mInsertKeyState
    End If

    Me.DrawCaret()
End Sub

Protected Overrides Sub OnMouseUp(ByVal mevent As System.Windows.Forms.MouseEventArgs)
    MyBase.OnMouseUp(mevent)

    Me.DrawCaret()
End Sub

Public Sub DrawCaret()
    Dim nHeight As Integer = 0
    Dim nWidth As Integer = 0

    If Me.SelectionFont IsNot Nothing Then
        nHeight = CInt(Me.SelectionFont.Height * Me.ZoomFactor)
    Else
        nHeight = CInt(Me.Font.Height * Me.ZoomFactor)
    End If

    If Not mInsertKeyState AndAlso Me.SelectionStart < Me.TextLength Then
        Dim p1 As Point = MyBase.GetPositionFromCharIndex(Me.SelectionStart)
        Dim p2 As Point = MyBase.GetPositionFromCharIndex(Me.SelectionStart + 1)

        nWidth = p2.X - p1.X
    End If

    CreateCaret(Me.Handle, IntPtr.Zero, nWidth, nHeight)
    ShowCaret(Me.Handle)
End Sub