net netcore2 netcore3 HtmlHelper扩展（checkboxlistfor)为例

　　　　　public static MvcHtmlString CheckBoxListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<SelectListItem> items, object htmlAttributes)
        {
            var result = new StringBuilder();
            string name = ExpressionHelper.GetExpressionText(expression);
            var modelData = ModelMetadata.FromLambdaExpression<TModel, TProperty>(expression, htmlHelper.ViewData).Model;
            List<string> currentValues = StringHelper.StringSplit<string>(modelData == null ? string.Empty : modelData.ToString());
            foreach (var item in items)
            {
                if (currentValues.Contains(item.Value))
                    result.AppendFormat("<label class=\"checkbox-inline checkbox-styled\"><input type=\"checkbox\" name=\"{0}\" value=\"{1}\" checked><span class=‘text‘>{2}</span></label>", name, item.Value, item.Text);
                else
                    result.AppendFormat("<label class=\"checkbox-inline checkbox-styled\"><input type=\"checkbox\" name=\"{0}\" value=\"{1}\"><span class=‘text‘>{2}</span></label>", name, item.Value, item.Text);
            }
            return MvcHtmlString.Create(result.ToString());
        }

　　　　public static IHtmlContent CheckBoxListFor<TModel, TProperty> (this IHtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<SelectListItem> items, object htmlAttributes)
        {
            var result = new StringBuilder();
            string name = ExpressionHelper.GetExpressionText(expression);
            var modelData = ExpressionMetadataProvider.FromLambdaExpression<TModel, TProperty>(expression, htmlHelper.ViewData, htmlHelper.MetadataProvider).Model;
            List<string> currentValues = StringHelper.StringSplit<string>(modelData == null ? string.Empty : modelData.ToString());
            foreach (var item in items)
            {
                if (currentValues.Contains(item.Value))
                    result.AppendFormat("<label class=\"checkbox-inline checkbox-styled\"><input type=\"checkbox\" name=\"{0}\" value=\"{1}\" checked><span class=‘text‘>{2}</span></label>", name, item.Value, item.Text);
                else
                    result.AppendFormat("<label class=\"checkbox-inline checkbox-styled\"><input type=\"checkbox\" name=\"{0}\" value=\"{1}\"><span class=‘text‘>{2}</span></label>", name, item.Value, item.Text);
            }
            return new HtmlString(result.ToString());
        }

 　　　　public static IHtmlContent CheckBoxListFor<TModel, TProperty> (this IHtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression, IEnumerable<SelectListItem> items, object htmlAttributes)
        {
            var result = new StringBuilder();
            ModelExpressionProvider modelExpressionProvider = (ModelExpressionProvider)htmlHelper.ViewContext.HttpContext.RequestServices.GetService(typeof(IModelExpressionProvider));
            var modelData = modelExpressionProvider.CreateModelExpression(htmlHelper.ViewData, expression).Model;
            var name=modelExpressionProvider.GetExpressionText(expression);

            List<string> currentValues = ((modelData == null) ? string.Empty : modelData.ToString()).Split<string>(Array.Empty<string>());

            foreach (var item in items)
            {
                if (currentValues.Contains(item.Value))
                    result.AppendFormat("<label class=\"checkbox-inline checkbox-styled\"><input type=\"checkbox\" name=\"{0}\" value=\"{1}\" checked><span class=‘text‘>{2}</span></label>", name, item.Value, item.Text);
                else
                    result.AppendFormat("<label class=\"checkbox-inline checkbox-styled\"><input type=\"checkbox\" name=\"{0}\" value=\"{1}\"><span class=‘text‘>{2}</span></label>", name, item.Value, item.Text);
            }
            return new HtmlString(result.ToString());
        }