How do I maintain RichText formatting (bold/italic/etc) when changing any one element?

Question

I have a rich text box that may contain a string that has elements of bold, italics, or even different fonts and sizes. If I select the entire string, including all of the d

Answer 1

In case you want also to change its font family and font size you could use this method: Hans is right, you have to iterate each character.

  private void ChangeFontStyleForSelectedText(string familyName, float? emSize, FontStyle? fontStyle, bool? enableFontStyle)
    {
        _maskChanges = true;
        try
        {
            int txtStartPosition = txtFunctionality.SelectionStart;
            int selectionLength = txtFunctionality.SelectionLength;
            if (selectionLength > 0)
                using (RichTextBox txtTemp = new RichTextBox())
                {
                    txtTemp.Rtf = txtFunctionality.SelectedRtf;
                    for (int i = 0; i < selectionLength; ++i)
                    {
                        txtTemp.Select(i, 1);
                        txtTemp.SelectionFont = RenderFont(txtTemp.SelectionFont, familyName, emSize, fontStyle, enableFontStyle);
                    }

                    txtTemp.Select(0, selectionLength);
                    txtFunctionality.SelectedRtf = txtTemp.SelectedRtf;
                    txtFunctionality.Select(txtStartPosition, selectionLength);
                }
        }
        finally
        {
            _maskChanges = false;
        }
    }

      /// 

    /// Changes a font from originalFont appending other properties
    /// 
    /// Original font of text
    /// Target family name
    /// Target text Size
    /// Target font style
    /// true when enable false when disable
    /// A new font with all provided properties added/removed to original font
    private Font RenderFont(Font originalFont, string familyName, float? emSize, FontStyle? fontStyle, bool? enableFontStyle)
    {
        if (fontStyle.HasValue && fontStyle != FontStyle.Regular && fontStyle != FontStyle.Bold && fontStyle != FontStyle.Italic && fontStyle != FontStyle.Underline)
            throw new System.InvalidProgramException("Invalid style parameter to ChangeFontStyleForSelectedText");

        Font newFont;
        FontStyle? newStyle = null;
        if (fontStyle.HasValue)
        {
            if (fontStyle.HasValue && fontStyle == FontStyle.Regular)
                newStyle = fontStyle.Value;
            else if (originalFont != null && enableFontStyle.HasValue && enableFontStyle.Value)
                newStyle = originalFont.Style | fontStyle.Value;
            else
                newStyle = originalFont.Style & ~fontStyle.Value;
        }

        newFont = new Font(!string.IsNullOrEmpty(familyName) ? familyName : originalFont.FontFamily.Name,
                            emSize.HasValue ? emSize.Value : originalFont.Size,
                            newStyle.HasValue ? newStyle.Value : originalFont.Style);
        return newFont;
    }

For more explanations you could go to: http://how-to-code-net.blogspot.ro/2014/01/how-to-make-custom-richtextbox-control.html