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我正在尝试构建一个 MVC 助手，用于使用我自己的参数构建一个 MultiSelectList。我基于我不久前构建的 SelectFor 助手。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，只有一个意义不同：forExpression将是IEnumerable的属性类型的泛型idExpression。此集合将用于“预选”列表项，并将作为表单上选定项的返回值。我（我认为）在这方面走得更远，但仍然很迷茫。

    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);
    }

帮助表示赞赏。

更新 感谢您的回答！泛型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！

4

1 回答 1

4

听起来你在要求Expression<Func<TModel, IEnumerable<TListItem>>.

您需要创建一个单独的通用参数并将其用于 ID 属性和 for 表达式：

public static HtmlString MultiSelectListFor<TModel, TKey, TListItem>(
    this HtmlHelper<TModel> htmlHelper,
    Expression<Func<TModel, IEnumerable<TKey>>> forExpression,
    IEnumerable<TListItem> enumeratedItems,
    Func<TListItem, TKey> idExpression,
于 2012-03-20T18:55:47.927 回答