Unable to Calculate Position within Owner-Draw Text
- by Jonathan Wood
I'm trying to use Visual Studio 2012 to create a Windows Forms application that can place the caret at the current position within a owner-drawn string. However, I've been unable to find a way to accurately calculate that position.
I've done this successfully before in C++. I've now tried numerous methods in C#. Originally, I tried using .NET classes to determine the correct position, but then I tried accessing the Windows API directly. In some cases, I came close, but after some time I still cannot place the caret accurately.
I've created a small test program and posted key parts below. I've also posted the entire project here.
The exact font used is not important to me; however, my application assumes a mono-spaced font. Any help is appreciated.
Form1.cs
This is my main form.
public partial class Form1 : Form
{
private string TestString;
private int AveCharWidth;
private int Position;
public Form1()
{
InitializeComponent();
TestString = "123456789012345678901234567890123456789012345678901234567890";
AveCharWidth = GetFontWidth();
Position = 0;
}
private void Form1_Load(object sender, EventArgs e)
{
Font = new Font(FontFamily.GenericMonospace, 12, FontStyle.Regular, GraphicsUnit.Pixel);
}
protected override void OnGotFocus(EventArgs e)
{
Windows.CreateCaret(Handle, (IntPtr)0, 2, (int)Font.Height);
Windows.ShowCaret(Handle);
UpdateCaretPosition();
base.OnGotFocus(e);
}
protected void UpdateCaretPosition()
{
Windows.SetCaretPos(Padding.Left + (Position * AveCharWidth), Padding.Top);
}
protected override void OnLostFocus(EventArgs e)
{
Windows.HideCaret(Handle);
Windows.DestroyCaret();
base.OnLostFocus(e);
}
protected override void OnPaint(PaintEventArgs e)
{
e.Graphics.DrawString(TestString, Font, SystemBrushes.WindowText,
new PointF(Padding.Left, Padding.Top));
}
protected override bool IsInputKey(Keys keyData)
{
switch (keyData)
{
case Keys.Right:
case Keys.Left:
return true;
}
return base.IsInputKey(keyData);
}
protected override void OnKeyDown(KeyEventArgs e)
{
switch (e.KeyCode)
{
case Keys.Left:
Position = Math.Max(Position - 1, 0);
UpdateCaretPosition();
break;
case Keys.Right:
Position = Math.Min(Position + 1, TestString.Length);
UpdateCaretPosition();
break;
}
base.OnKeyDown(e);
}
protected int GetFontWidth()
{
int AverageCharWidth = 0;
using (var graphics = this.CreateGraphics())
{
try
{
Windows.TEXTMETRIC tm;
var hdc = graphics.GetHdc();
IntPtr hFont = this.Font.ToHfont();
IntPtr hOldFont = Windows.SelectObject(hdc, hFont);
var a = Windows.GetTextMetrics(hdc, out tm);
var b = Windows.SelectObject(hdc, hOldFont);
var c = Windows.DeleteObject(hFont);
AverageCharWidth = tm.tmAveCharWidth;
}
catch
{
}
finally
{
graphics.ReleaseHdc();
}
}
return AverageCharWidth;
}
}
Windows.cs
Here are my Windows API declarations.
public static class Windows
{
[Serializable, StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
public struct TEXTMETRIC
{
public int tmHeight;
public int tmAscent;
public int tmDescent;
public int tmInternalLeading;
public int tmExternalLeading;
public int tmAveCharWidth;
public int tmMaxCharWidth;
public int tmWeight;
public int tmOverhang;
public int tmDigitizedAspectX;
public int tmDigitizedAspectY;
public short tmFirstChar;
public short tmLastChar;
public short tmDefaultChar;
public short tmBreakChar;
public byte tmItalic;
public byte tmUnderlined;
public byte tmStruckOut;
public byte tmPitchAndFamily;
public byte tmCharSet;
}
[DllImport("user32.dll")]
public static extern bool CreateCaret(IntPtr hWnd, IntPtr hBitmap, int nWidth, int nHeight);
[DllImport("User32.dll")]
public static extern bool SetCaretPos(int x, int y);
[DllImport("User32.dll")]
public static extern bool DestroyCaret();
[DllImport("User32.dll")]
public static extern bool ShowCaret(IntPtr hWnd);
[DllImport("User32.dll")]
public static extern bool HideCaret(IntPtr hWnd);
[DllImport("gdi32.dll", CharSet = CharSet.Auto)]
public static extern bool GetTextMetrics(IntPtr hdc, out TEXTMETRIC lptm);
[DllImport("gdi32.dll")]
public static extern IntPtr SelectObject(IntPtr hdc, IntPtr hgdiobj);
[DllImport("GDI32.dll")]
public static extern bool DeleteObject(IntPtr hObject);
}
