或
public static class StringBuilderExtensions
{
public static StringBuilder AppendFormattedLine(this StringBuilder @this， string format， prams object[] args) => @this.AppendFormat(format， args).AppendLine();
public static StringBuilder AppendLineWhen(this StringBuilder @this， Func<bool> predicate， string value) =>
predicate()
? @this.AppendLine(value)
: @this;
public static StringBuilder AppendWhen(this StringBuilder @this， Func<bool> predicate， Func<StringBuilder， StringBuilder> fn) =>
predicate()
? fn(@this)
: @this;
}
private static string BuilderSelectBox(IDictionary<int， string> options， string id， bool includeUnknown)
{
var html = new StringBuilder()
.AppendFormattedLine("<select id="{0}" name="{0}">"， id)
.AppendWhen(
() => includeUnknown，
sb => sb.AppendLine(" <option>Unknown</option>")
);
foreach(var opt in options)
{
html.AppendFormattedLine(" <option value="{0}">{1}</option>"， opt.Key， opt.Value);
}
html.AppendLine("</select>");
return html.ToString();
}
　　以上，
　　foreach(var opt in options)
　　{
　　html.AppendFormattedLine(" <option value="{0}">{1}</option>"， opt.Key， opt.Value);
　　}
　　对遍历语句进行封装，扩展StringBuilder，终：
public static class StringBuilderExtensions
{
public static StringBuilder AppendFormattedLine(this StringBuilder @this， string format， prams object[] args) => @this.AppendFormat(format， args).AppendLine();
public static StringBuilder AppendLineWhen(this StringBuilder @this， Func<bool> predicate， string value) =>
predicate()
? @this.AppendLine(value)
: @this;
public static StringBuilder AppendWhen(this StringBuilder @this， Func<bool> predicate， Func<StringBuilder， StringBuilder> fn) =>
predicate()
? fn(@this)
: @this;
public static StringBuilder AppendSequence<T>(this StringBuilder @this， IEnumerable<T> seq， Func<StringBuilder， T， StringBuilder> fn) => seq.Aggregate(@this， fn);
}
private static string BuilderSelectBox(IDictionary<int， string> options， string id， bool includeUnknown)
{
var html = new StringBuilder()
.AppendFormattedLine("<select id="{0}" name="{0}">"， id)
.AppendWhen(
() => includeUnknown，
sb => sb.AppendLine(" <option>Unknown</option>")
)
.AppendSequence(options， (sb， opt) => sb.AppendFormattedLine(" <option value="{0}">{1}</option>"， opt.Key， opt.Value))
.AppendLine("</select>")
.ToString();
}