MainWindow.xaml.cs

using System.Diagnostics;
using System.Text;
using System.Windows;

namespace WpfApp1
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void btnConvert_Click(object sender, RoutedEventArgs e)
        {
            string SVGInput = txtInput.Text.Trim();
            string BootstrapClassValue = GetClassAttributeValue(SVGInput).Split(' ')[1] + "-nav-menu";

            string NewSvgData = SVGInput.Replace(Environment.NewLine, "")
                .Replace('"', '\'')
                .Replace("<", "%3C")
                .Replace(">", "%3E")
                .Replace("currentColor", txtIconForegroundColor.Text.Trim());

            StringBuilder cssOutputBuilder = new StringBuilder();
            cssOutputBuilder.AppendLine("." + BootstrapClassValue + " {")
                .AppendLine("\tbackground-image: url(\"data:image/svg+xml," + NewSvgData + "\");")
                .AppendLine("}");

            txtOutputCSS.Text = cssOutputBuilder.ToString();

            StringBuilder navMenuOutputBuilder = new StringBuilder();
            navMenuOutputBuilder.AppendLine("<div class=\"nav-item px-3\">")
                .AppendLine("\t<NavLink class=\"nav-link\" href=\"pagename\">")
                .AppendLine("\t\t<span class=\"bi " + BootstrapClassValue + "\" aria-hidden=\"true\"></span> Your Page Name")
                .AppendLine("\t</NavLink>")
                .AppendLine("</div>");

            txtOutputNavMenuTemplate.Text = navMenuOutputBuilder.ToString();
        }

        public static string GetClassAttributeValue(string html)
        {
            string classAttribute = "class=\"";
            int classIndex = html.IndexOf(classAttribute);
            if (classIndex == -1)
            {
                return string.Empty;
            }

            int startIndex = classIndex + classAttribute.Length;
            int endIndex = html.IndexOf("\"", startIndex);

            if (endIndex == -1)
            {
                return string.Empty;
            }

            return html.Substring(startIndex, endIndex - startIndex);
        }

        private void btnVisitBootstrapIconsPage_Click(object sender, RoutedEventArgs e)
        {
            Process.Start(new ProcessStartInfo
            {
                FileName = "https://icons.getbootstrap.com/",
                UseShellExecute = true
            });
        }

        private void btnCopyCSSToClipboard_Click(object sender, RoutedEventArgs e)
        {
            Clipboard.SetText(txtOutputCSS.Text);
            MessageBox.Show("CSS code copied to clipboard!");
        }

        private void btnCopyHTMLToClipboard_Click(object sender, RoutedEventArgs e)
        {
            Clipboard.SetText(txtOutputNavMenuTemplate.Text);
            MessageBox.Show("HTML code copied to clipboard!");
        }
    }
}