WinForm richtextbox deep line spacing and character spacing – Photoshop

Line Spacing

You can send EM_SETPARAFORMAT message to the rich text box control and pass PARAFORMAT2 as lparam. To control line spacing, you should set the PFM_LINESPACING flag in the dwMask member and set bLineSpacingRule and dyLineSpacing members of PARAFORMAT2 to suitable values based on your requirements.

Since you need fine-tuning for line spacing, it seems 4 is suitable for bLineSpacingRule and then you can set dyLineSpacing to any value in twip unit. For more information about available options for bLineSpacingRule, read PARAFORMAT2 documentations.

public void SetSelectionLineSpacing(byte bLineSpacingRule, int dyLineSpacing)
{
    PARAFORMAT2 format = new PARAFORMAT2();
    format.cbSize = Marshal.SizeOf(format);
    format.dwMask = PFM_LINESPACING;
    format.dyLineSpacing = dyLineSpacing;
    format.bLineSpacingRule = bLineSpacingRule;
    SendMessage(this.Handle, EM_SETPARAFORMAT, SCF_SELECTION, ref format);
}

Character Spacing

Based on documentation of sSpacing in CHARFORMAT2, setting character spacing has no effect on the text displayed by a rich edit control.

Code

public class ExRichText : RichTextBox
{
    [DllImport("user32.dll", EntryPoint = "SendMessage", CharSet = CharSet.Auto)]
    private static extern IntPtr SendMessage(IntPtr hWnd, Int32 msg, 
                                             Int32 wParam, ref PARAFORMAT2 lParam);

    private const int SCF_SELECTION = 1;
    public const int PFM_LINESPACING = 256;
    public const int EM_SETPARAFORMAT = 1095;

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    public struct PARAFORMAT2
    {
        public int cbSize;
        public uint dwMask;
        public Int16 wNumbering;
        public Int16 wReserved;
        public int dxStartIndent;
        public int dxRightIndent;
        public int dxOffset;
        public Int16 wAlignment;
        public Int16 cTabCount;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
        public int[] rgxTabs;
        public int dySpaceBefore;
        public int dySpaceAfter;
        public int dyLineSpacing;
        public Int16 sStyle;
        public byte bLineSpacingRule;
        public byte bOutlineLevel;
        public Int16 wShadingWeight;
        public Int16 wShadingStyle;
        public Int16 wNumberingStart;
        public Int16 wNumberingStyle;
        public Int16 wNumberingTab;
        public Int16 wBorderSpace;
        public Int16 wBorderWidth;
        public Int16 wBorders;
    }

    public void SetSelectionLineSpacing(byte bLineSpacingRule, int dyLineSpacing)
    {
        PARAFORMAT2 format = new PARAFORMAT2();
        format.cbSize = Marshal.SizeOf(format);
        format.dwMask = PFM_LINESPACING;
        format.dyLineSpacing = dyLineSpacing;
        format.bLineSpacingRule = bLineSpacingRule;
        SendMessage(this.Handle, EM_SETPARAFORMAT, SCF_SELECTION, ref format);
    }
}