c#のラムダ式などを使って閉じ忘れのないhtmlビルダーを作りたい ref: http://qiita.com/Implem/items/a141e57607edcebd5ff2

Extensions.cs

using System.Linq;
using System.Text;
namespace Implem.Sample
{
    public static class Extensions
    {
        // 連続Append用拡張メソッド
        public static void Append(this StringBuilder stringBuilder, params string[] strings)
        {
            strings.ForEach(str => stringBuilder.Append(str));
        }
    }
}

HtmlBuilder.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
namespace Implem.Sample
{
    public class HtmlBuilder
    {
        private StringBuilder html = new StringBuilder();
        private List<string> closeTagCollection = new List<string>();

        private enum AttributePropertyTypes : int
        {
            Name = 0,
            Value = 1
        }

        // タグ追加
        public HtmlBuilder Append(string tag, int closeLevel = 0, params string[] attributes)
        {
            html.Append("<", tag);
            attributes
                .Select((o, i) => new { Value = o, PropertyType = (AttributePropertyTypes)(i % 2) })
                .ForEach(data =>
                {
                    switch (data.PropertyType)
                    {
                        case AttributePropertyTypes.Name:

                            // 偶数はプロパティ名
                            html.Append(" " + data.Value); break;

                        case AttributePropertyTypes.Value:

                            // 奇数は値
                            html.Append("=\"", data.Value, "\"");break;
                    }
                });
            if (closeLevel > 0)
            {
                html.Append(" />");
                if (closeLevel >= 2)
                {
                    AppendCloseTagCollection(closeLevel - 1);
                }
            }
            else
            {
                html.Append(">");
                closeTagCollection.Insert(0, "</" + tag + ">");
            }
            return this;
        }

        // 属性付きタグ追加の間に別の処理
        public HtmlBuilder Append(string tag, string[] attributes, Action action)
        {
            Append(tag, attributes: attributes);
            action();
            AppendClose();
            return this;
        }

        // タグ追加の間に別の処理
        public HtmlBuilder Append(string tag, Action action)
        {
            Append(tag, attributes: new string[] { });
            action();
            AppendClose();
            return this;
        }

        // id, class属性付きタグ追加の間に別の処理
        public HtmlBuilder Append(string tag, string controlId, string cssClass, Action action)
        {
            Append(tag, attributes: new string[] { "id", controlId, "class", cssClass });
            action();
            AppendClose();
            return this;
        }

        // class属性付きタグ追加の間に別の処理
        public HtmlBuilder Append(string tag, string cssClass, Action action)
        {
            Append(tag, attributes: new string[] { "class", cssClass });
            action();
            AppendClose();
            return this;
        }

        // テキスト追加とクローズ
        public HtmlBuilder AppendText(string str, int closeLevel = 0)
        {
            html.Append(str);
            AppendCloseTagCollection(closeLevel);
            return this;
        }

        // クローズ
        public HtmlBuilder AppendClose(int closeLevel = 1)
        {
            AppendCloseTagCollection(closeLevel);
            return this;
        }

        // 全部クローズ
        public HtmlBuilder AppendCloseAll()
        {
            AppendCloseTagCollection(-1);
            return this;
        }

        // 全部クローズしてHtml出力
        public override string ToString()
        {
            // html出力前に閉じ忘れしないよう全部クローズ
            AppendCloseTagCollection();
            return html.ToString();
        }

        // クローズ処理
        private void AppendCloseTagCollection(int closeLevel = -1)
        {
            if (closeTagCollection.Count() > 0)
            {
                if (closeLevel == -1)
                {
                    // -1 : 全部閉じる
                    html.Append(string.Join("", closeTagCollection));
                    closeTagCollection.Clear();
                }
                else
                {
                    // n : n個閉じる
                    html.Append(string.Join(string.Empty, closeTagCollection.Take(closeLevel)));
                    closeTagCollection = closeTagCollection.Skip(closeLevel).ToList<string>();
                }
            }
        }
    }
}

output.html

<div><p class="content">hoge</p></div>
<div><a id="hoge" class="fuga" href="http://qiita.com/">foo</a></div>
<table><tr><th>1-1</th><th>1-2</th></tr><tr><td>2-1</td><td>2-2</td></tr></table>

Program.cs

using System;
using System.Linq;
namespace Implem.Sample
{
    class Program
    {
        static void Main(string[] args)
        {
            // 使用例1
            var htmlBuilder1 = new HtmlBuilder();
            Console.WriteLine(
                htmlBuilder1.Append("div", () =>
                    htmlBuilder1.Append("p", "content", () =>
                        htmlBuilder1.AppendText("hoge"))));

            // 使用例2
            var htmlBuilder2 = new HtmlBuilder();
            Console.WriteLine(
                htmlBuilder2.Append("div", () =>
                    htmlBuilder2.Append("a", new string[]
                    {
                        "id", "hoge",
                        "class", "fuga",
                        "href", "http://qiita.com/"
                    },
                    () =>
                        htmlBuilder2.AppendText("foo"))));

            // 使用例3
            var htmlBuilder3 = new HtmlBuilder();
            Console.WriteLine(
                htmlBuilder3.Append("table", () =>
                    Enumerable.Range(1, 2).ForEach(x =>
                        htmlBuilder3.Append("tr", () =>
                            Enumerable.Range(1, 2).ForEach(y =>
                            {
                                if (x == 1)
                                {
                                    htmlBuilder3.Append("th", () =>
                                        htmlBuilder3.AppendText(x + "-" + y));
                                }
                                else
                                {
                                    htmlBuilder3.Append("td", () =>
                                        htmlBuilder3.AppendText(x + "-" + y));
                                }
                            })))));
        }
    }
}