WinForms: SelectionCharOffset and FontSize

在内部，底层的MFC富编辑控件将y偏移量存储在twtip中，twtip是1/1440英寸或1/20打印机点。然而，由于某些原因，SelectionCharOffset隐藏了这种精度水平，而是以像素为单位返回，其精度大约低20倍。如果twips足够精确以满足您的需求，您可以创建自己的扩展方法来获取和设置twips中的SelectionCharOffset，以参考源为模型:

public static class RichTextBoxNativeMethods
{
    private static CHARFORMAT2 GetCharFormat(this RichTextBox richTextBox, bool fSelection)
    {
        // Either CHARFORMAT or CHARFORMAT2 can be used as long as the cbSize is set correctly.
        // https://msdn.microsoft.com/en-us/library/windows/desktop/bb774230.aspx
        // Force handle creation
        if (!richTextBox.IsHandleCreated)
        {
            var handle = richTextBox.Handle;
        }
        var cf2 = new CHARFORMAT2();
        UnsafeNativeMethods.SendMessage(new HandleRef(richTextBox, richTextBox.Handle), RichTextBoxConstants.EM_GETCHARFORMAT, fSelection ? RichTextBoxConstants.SCF_SELECTION : RichTextBoxConstants.SCF_DEFAULT, cf2);
        return cf2;
    }
    public static void SetSelectionCharOffsetInTwips(this RichTextBox richTextBox, int offsetInTwips)
    {
        // Adapted from http://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/RichTextBox.cs,7765c7694b8e433f
        // Force handle creation
        if (!richTextBox.IsHandleCreated)
        {
            var handle = richTextBox.Handle;
        }
        var cf2 = new CHARFORMAT2();
        cf2.dwMask = RichTextBoxConstants.CFM_OFFSET;
        cf2.yOffset = offsetInTwips;
        UnsafeNativeMethods.SendMessage(new HandleRef(richTextBox, richTextBox.Handle), RichTextBoxConstants.EM_SETCHARFORMAT, RichTextBoxConstants.SCF_SELECTION, cf2);
    }
    public static int GetSelectionCharOffsetInTwips(this RichTextBox richTextBox)
    {
        // Adapted from http://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/RichTextBox.cs,7765c7694b8e433f
        int selCharOffset = 0;
        var cf2 = richTextBox.GetCharFormat(true);
        if ((cf2.dwMask & RichTextBoxConstants.CFM_OFFSET) != 0)
        {
            selCharOffset = cf2.yOffset;
        }
        else
        {
            // The selection contains characters of different offsets,
            // so we just return the offset of the first character.
            selCharOffset = cf2.yOffset;
        }
        return selCharOffset;
    }
}
public static class NativeMethods
{
    public const int WM_USER = 0x0400;
}
[StructLayout(LayoutKind.Sequential, Pack = 4)]
public class CHARFORMAT2
{
    // http://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/NativeMethods.cs,acde044a28b57a48
    // http://pinvoke.net/default.aspx/Structures/CHARFORMAT2.html
    public int cbSize = Marshal.SizeOf(typeof(CHARFORMAT2));
    public int dwMask;
    public int dwEffects;
    public int yHeight;
    public int yOffset;
    public int crTextColor;
    public byte bCharSet;
    public byte bPitchAndFamily;
    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 32)]
    public string szFaceName;
    public short wWeight;
    public short sSpacing;
    public int crBackColor;
    public int lcid;
    public int dwReserved;
    public short sStyle;
    public short wKerning;
    public byte bUnderlineType;
    public byte bAnimation;
    public byte bRevAuthor;
    public byte bReserved1;
}
public static class RichTextBoxConstants
{
    // Trimmed down from
    // http://referencesource.microsoft.com/#System.Windows.Forms/winforms/Managed/System/WinForms/RichTextBoxConstants.cs,31b52ac41e96a888
    /* EM_SETCHARFORMAT wparam masks */
    /* EM_SETCHARFORMAT wparam masks */
    internal const int SCF_SELECTION = 0x0001;
    internal const int SCF_WORD = 0x0002;
    internal const int SCF_DEFAULT = 0x0000;   // set the default charformat or paraformat
    internal const int SCF_ALL = 0x0004;   // not valid with SCF_SELECTION or SCF_WORD
    internal const int SCF_USEUIRULES = 0x0008;   // modifier for SCF_SELECTION; says that
                                                // the format came from a toolbar, etc. and
                                                // therefore UI formatting rules should be
                                                // used instead of strictly formatting the
                                                // selection.
    internal const int EM_SETCHARFORMAT = (NativeMethods.WM_USER + 68);
    internal const int EM_GETCHARFORMAT = (NativeMethods.WM_USER + 58);
    internal const int CFM_BACKCOLOR = 0x04000000;
    internal const int CFM_OFFSET = 0x10000000;
    /* NOTE: CFE_AUTOCOLOR and CFE_AUTOBACKCOLOR correspond to CFM_COLOR and
       CFM_BACKCOLOR, respectively, which control them */
    internal const int CFE_AUTOBACKCOLOR = CFM_BACKCOLOR;
}