迷失在仿制药和反思酱中

我正在尝试构建一个MVC助手，用于使用我自己的参数构建MultiSelectList。我把它建立在一个SelectFor助手的基础上。SelectFor看起来像这样：

    public delegate object Property<T>(T property);
    public delegate object Property<T, K>(T property, K propertyKey);
    public static HtmlString SelectFor<TModel, TProperty, TListItem>(
        this HtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, TProperty>> forExpression,
        IEnumerable<TListItem> enumeratedItems,
        Property<TListItem> idExpression,
        Property<TListItem> displayExpression,
        Property<TListItem> titleExpression,
        object htmlAttributes,
        bool blankFirstLine) where TModel : class
    {
        //initialize values
        var metaData = ModelMetadata.FromLambdaExpression(forExpression, 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";
        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}'"",
                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);
    }

MultiSelectFor只是略有不同，有一个显著性差异：forExpression将是idExpression属性类型的通用IEnumerable。此集合将用于"预选"列表项目，并将作为表单上所选项目的返回值。我（我认为）在这方面做得更远，但仍然很失落。

    public static HtmlString MultiSelectListFor<TModel, TProperty, TProperty, TPropertyKey>(
        this HtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, IEnumerable<TPropertyKey>>> forExpression,
        IEnumerable<TProperty> enumeratedItems,
        Property<TProperty, TPropertyKey> idExpression,
        Property<TProperty> displayExpression,
        Property<TProperty> titleExpression,
        object htmlAttributes) where TModel : class
    {
        //initialize values
        var metaData = ModelMetadata.FromLambdaExpression(forExpression, htmlHelper.ViewData);
        var propertyName = metaData.PropertyName;
        var propertyValue = htmlHelper.ViewData.Eval(propertyName).ToStringOrEmpty();
        var enumeratedType = typeof(TProperty);
        //build the select tag
        var returnText = string.Format("<select multiple='"multiple'" 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 (TProperty listItem in enumeratedItems)
        {
            //this part here needs to change:
            var idValue = ???.ToStringOrEmpty();
            var displayValue = displayExpression(listItem).ToStringOrEmpty();
            var titleValue = titleExpression(listItem).ToStringOrEmpty();
            returnText += string.Format("<option value='"{0}'" title='"{1}'"",
                HttpUtility.HtmlEncode(idValue), HttpUtility.HtmlEncode(titleValue));
            if (propertyValue.Contains(idValue))
            {
                returnText += " selected='"selected'"";
            }
            returnText += string.Format(">{0}</option>'n", displayValue);
        }
        //close the select tag
        returnText += "</select>";
        return new HtmlString(returnText);
    }

感谢您的帮助。

更新谢谢你的回答！Generics有时会让我感到困惑。完整的解决方案如下：

    public delegate object Property<T>(T property);
    public static HtmlString MultiSelectListFor<TModel, TKey, TProperty>(
        this HtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, IEnumerable<TKey>>> forExpression,
        IEnumerable<TProperty> enumeratedItems,
        Func<TProperty, TKey> idExpression,
        Property<TProperty> displayExpression,
        Property<TProperty> titleExpression,
        object htmlAttributes) where TModel : class
    {
        //initialize values
        var metaData = ModelMetadata.FromLambdaExpression(forExpression, htmlHelper.ViewData);
        var propertyName = metaData.PropertyName;
        var propertyValue = htmlHelper.ViewData.Eval(propertyName).ToStringOrEmpty();
        var enumeratedType = typeof(TProperty);
        //build the select tag
        var returnText = string.Format("<select multiple='"multiple'" 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 (TProperty 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}'"",
                HttpUtility.HtmlEncode(idValue), HttpUtility.HtmlEncode(titleValue));
            if (propertyValue.Contains(idValue))
            {
                returnText += " selected='"selected'"";
            }
            returnText += string.Format(">{0}</option>'n", displayValue);
        }
        //close the select tag
        returnText += "</select>";
        return new HtmlString(returnText);
    }
    public static HtmlString MultiSelectListFor<TModel, TKey, TProperty>(
        this HtmlHelper<TModel> htmlHelper,
        Expression<Func<TModel, IEnumerable<TKey>>> forExpression,
        IEnumerable<TProperty> enumeratedItems,
        Func<TProperty, TKey> idExpression,
        Property<TProperty> displayExpression,
        Property<TProperty> titleExpression) where TModel : class
    {
        return htmlHelper.MultiSelectListFor(forExpression, enumeratedItems, idExpression, displayExpression, titleExpression, null);
    }

再次感谢您的帮助！