Simple Markdown Sketch

SimpleMarkdown.cs

using System.Collections.Generic;

namespace SimpleMarkdown;

class Testing
{
    public static void Test()
    {
        // either use .With or set MarkdownBuilder.DefaultParseMode to globally set it once
        string nice = "Hey!".Bold().Link("https://google.com/").With(ParseMode.Markdown);
        Console.WriteLine(nice); // => [**Hey!**](https://google.com/)
    }
}

enum ParseMode
{
    Html,
    Markdown,
}

abstract class MarkdownElement
{
    public abstract string Apply(string content, ParseMode mode);
}

class BoldMarkdown : MarkdownElement
{
    public override string Apply(string content, ParseMode mode) => mode switch
    {
        ParseMode.Html => $"<b>{content}</b>",
        ParseMode.Markdown => $"**{content}**",
        _ => content,
    };
}

class LinkMarkdown(string url) : MarkdownElement
{
    public string Url { get; } = url;

    public override string Apply(string content, ParseMode mode) => mode switch
    {
        ParseMode.Html => $"<a href={Url}>{content}</a>",
        ParseMode.Markdown => $"[{content}]({Url})",
        _ => content,
    };
}

static class MarkdownExtensions
{
    public static MarkdownBuilder Bold(this string content) => new MarkdownBuilder(content).Bold();
    public static MarkdownBuilder Bold(this MarkdownBuilder builder)
    {
        if (!builder.Elements.OfType<BoldMarkdown>().Any())
            builder.Elements.Add(new BoldMarkdown());

        return builder;
    }

    public static MarkdownBuilder Link(this string content, string url) => new MarkdownBuilder(content).Link(url);
    public static MarkdownBuilder Link(this MarkdownBuilder builder, string url)
    {
        if (!builder.Elements.OfType<LinkMarkdown>().Any())
            builder.Elements.Add(new LinkMarkdown(url));

        return builder;
    }

    public static MarkdownBuilder With(this MarkdownBuilder builder, ParseMode mode)
    {
        builder.Mode = mode;
        return builder;
    }
}

class MarkdownBuilder(string content)
{
    public static ParseMode DefaultParseMode = ParseMode.Html;

    public string Content { get; } = content;
    public ParseMode? Mode { get; set; } = null;
    public ICollection<MarkdownElement> Elements { get; } = [];

    public static implicit operator string(MarkdownBuilder builder) => builder.ToString();

    public static implicit operator MarkdownBuilder(string text) => new(text);

    public override string ToString() => ToString(null);

    public string ToString(ParseMode? mode = null)
    {
        string result = Content;

        foreach (MarkdownElement element in Elements)
            result = element.Apply(result, mode ?? Mode ?? DefaultParseMode);

        return result;
    }
}