使用Linq表达式在扩展方法中有趣(？)

public static HtmlString SelectFor<TModel,TProperty,TListItem>(
        this HtmlHelper<TModel> htmlHelper,Expression<Func<TModel,TProperty>> expression,IEnumerable<TListItem> enumeratedItems,string idPropertyName,string displayPropertyName,string titlePropertyName,object htmlAttributes) where TModel : class
    {
        //initialize values
        var metaData = ModelMetadata.FromLambdaExpression(expression,htmlHelper.ViewData);
        var propertyName = metaData.PropertyName;
        var propertyValue = htmlHelper.ViewData.Eval(propertyName).ToStringOrEmpty();
        var enumeratedType = typeof(TListItem);

        //build the select tag
        var returnText = string.Format("<select id="{0}" name="{0}"",HttpUtility.HtmlEncode(propertyName));
        if (htmlAttributes != null)
        {
            foreach (var kvp in htmlAttributes.GetType().GetProperties()
             .ToDictionary(p => p.Name,p => p.GetValue(htmlAttributes,null)))
            {
                returnText += string.Format(" {0}="{1}"",HttpUtility.HtmlEncode(kvp.Key),HttpUtility.HtmlEncode(kvp.Value.ToStringOrEmpty()));
            }
        }
        returnText += ">n";

        //build the options tags
        foreach (TListItem listItem in enumeratedItems)
        {
            var idValue = enumeratedType.GetProperties()
             .FirstOrDefault(p => p.Name == idPropertyName)
             .GetValue(listItem,null).ToStringOrEmpty();
            var titleValue = enumeratedType.GetProperties()
             .FirstOrDefault(p => p.Name == titlePropertyName)
             .GetValue(listItem,null).ToStringOrEmpty();
            var displayValue = enumeratedType.GetProperties()
             .FirstOrDefault(p => p.Name == displayPropertyName)
             .GetValue(listItem,null).ToStringOrEmpty();
            returnText += string.Format("<option value="{0}" title="{1}"",HttpUtility.HtmlEncode(idValue),HttpUtility.HtmlEncode(titleValue));
            if (idValue == propertyValue)
            {
                returnText += " selected="selected"";
            }
            returnText += string.Format(">{0}</option>n",displayValue);
        }

        //close the select tag
        returnText += "</select>";
        return new HtmlString(returnText);
    }

……这很有效,但有时我想走得更远.我想自定义这个野兽的id,display和title piece,而不必写出html.例如,如果我在模型中有一些类,如下所示：

public class item
{
    public int itemId { get; set; }
    public string itemName { get; set; }
    public string itemDescription { get; set; }
}

public class model
{
    public IEnumerable<item> items { get; set; }
    public int itemId { get; set; }
}

在我看来,我可以写：

@Html.SelectFor(m => m.itemId,Model.items,"itemId","itemName","itemDescription",null)

…我会得到一个带有标题属性等的精彩下拉列表.只要枚举项具有与我想要显示它们完全相同的属性,这就很棒了.但我真正喜欢的是：

@Html.SelectFor(m => m.itemId,id=>id.itemId,disp=>disp.itemName,title=>title.itemName + " " + title.itemDescription,null)

…在这种情况下,选项上的title属性是itemName属性和itemDescription属性的串联.我承认lambda表达式的元级别,Linq函数让我有点头晕.有人能指出我正确的方向吗？

最终结果对于那些好奇的人,以下代码使我可以使用lambda表达式完全控制选择列表的ID,Title和DisplayText属性：

public static HtmlString SelectFor<TModel,TProperty>> forExpression,Attribute<TListItem> idExpression,Attribute<TListItem> displayExpression,Attribute<TListItem> titleExpression,object htmlAttributes,bool blankFirstLine) where TModel : class
    {
        //initialize values
        var metaData = ModelMetadata.FromLambdaExpression(forExpression,HttpUtility.HtmlEncode(kvp.Value.ToStringOrEmpty()));
            }
        }
        returnText += ">n";

        if (blankFirstLine)
        {
            returnText += "<option value=""></option>";
        }

        //build the options tags
        foreach (TListItem listItem in enumeratedItems)
        {
            var idValue = idExpression(listItem).ToStringOrEmpty();
            var displayValue = displayExpression(listItem).ToStringOrEmpty();
            var titleValue = titleExpression(listItem).ToStringOrEmpty();
            returnText += string.Format("<option value="{0}" title="{1}"",displayValue);
        }

        //close the select tag
        returnText += "</select>";
        return new HtmlString(returnText);
    }

    public delegate object Attribute<T>(T listItem);