Updated version of @Html.ValidationSummary to add the validation message source to the list items

CunningValidationSummaryHelper.cs

  using System;
  using System.Collections.Generic;
  using System.Linq;
  using System.Text;
  using System.Web.Mvc;

  // Use in views by calling `@Html.CunningValidationSummary(...)` instead of `@Html.ValidationSummary(...)`
  // It will give an attribute of `data-valmsg-source` on each list item in the validation summary with a value of the prop the error relates to.
  // Then you can use js to scroll to the field in question.
  //
  // eg. 
  // <div data-valmsg-summary="true" class="validation-summary-errors">
  //   <ul>
  //     <li data-valmsg-source="FundingSources">The funding sources must be $100,000</li>
  //   </ul>
  // </div>  
  //
  // See also: https://github.com/mono/aspnetwebstack/blob/master/src/System.Web.Mvc/Html/ValidationExtensions.cs
  public static class CunningValidationSummaryHelper
  {
    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper)
    {
      return htmlHelper.CunningValidationSummary(false);
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, bool excludePropertyErrors)
    {
      return htmlHelper.CunningValidationSummary(excludePropertyErrors, null);
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, string message)
    {
      return htmlHelper.CunningValidationSummary(false, message, null);
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, bool excludePropertyErrors, string message)
    {
      return htmlHelper.CunningValidationSummary(excludePropertyErrors, message, null);
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, string message, object htmlAttributes)
    {
      return htmlHelper.CunningValidationSummary(false, message, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, bool excludePropertyErrors, string message, object htmlAttributes)
    {
      return htmlHelper.CunningValidationSummary(excludePropertyErrors, message, HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes));
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, string message, IDictionary<string, object> htmlAttributes)
    {
      return htmlHelper.CunningValidationSummary(false, message, htmlAttributes);
    }

    public static MvcHtmlString CunningValidationSummary(this HtmlHelper htmlHelper, bool excludePropertyErrors, string message, IDictionary<string, object> htmlAttributes)
    {
      if (htmlHelper == null)
      {
        throw new ArgumentNullException("htmlHelper");
      }

      var formContextForClientValidation = GetFormContextForClientValidation(htmlHelper.ViewContext);
      if (htmlHelper.ViewData.ModelState.IsValid)
      {
        if (formContextForClientValidation == null)
        {
          return null;
        }

        if (htmlHelper.ViewContext.UnobtrusiveJavaScriptEnabled && excludePropertyErrors)
        {
          return null;
        }
      }

      string str;
      if (!string.IsNullOrEmpty(message))
      {
        var tagBuilder = new TagBuilder("span");
        tagBuilder.SetInnerText(message);
        str = tagBuilder.ToString(TagRenderMode.Normal) + Environment.NewLine;
      }
      else
      {
        str = null;
      }

      var stringBuilder = new StringBuilder();
      var tagBuilder2 = new TagBuilder("ul");
      Dictionary<string, ModelState> enumerable = null; // This is probably a shit way to get the name
      if (excludePropertyErrors)
      {
        // Not sure if we care about adding `data-valmsg-source` in this case?
        ModelState modelState;
        htmlHelper.ViewData.ModelState.TryGetValue(htmlHelper.ViewData.TemplateInfo.HtmlFieldPrefix, out modelState);
        if (modelState != null)
        {
          enumerable = new Dictionary<string, ModelState> { { string.Empty, modelState } };
        }
      }
      else
      {
        if (htmlHelper.ViewData.ModelState != null && htmlHelper.ViewData.ModelState.Any())
        {
          enumerable = htmlHelper.ViewData.ModelState.ToDictionary(modelState => modelState.Key, modelState => modelState.Value);
        } 
      }
            
      if (enumerable != null)
      {
        // `current` is a field on the submitted form, we want to get its id or name attribute.
        foreach (var current in enumerable)
        {
          // `current2` is an error associated with a field on the form
          foreach (var current2 in current.Value.Errors)
          {
            var userErrorMessageOrDefault = GetUserErrorMessageOrDefault(current2);
            if (!string.IsNullOrEmpty(userErrorMessageOrDefault))
            {
              var tagBuilder3 = new TagBuilder("li");
              
              var nameOrId = htmlHelper.ViewContext.ViewData.TemplateInfo.GetFullHtmlFieldName(current.Key);
              tagBuilder3.MergeAttribute("data-valmsg-source", nameOrId);
              
              tagBuilder3.SetInnerText(userErrorMessageOrDefault);
              stringBuilder.AppendLine(tagBuilder3.ToString(TagRenderMode.Normal));
            }
          }
        }
      }

      if (stringBuilder.Length == 0)
      {
        stringBuilder.AppendLine("<li style=\"display:none\"></li>");
      }

      tagBuilder2.InnerHtml = stringBuilder.ToString();

      var tagBuilder4 = new TagBuilder("div");
      tagBuilder4.MergeAttributes(htmlAttributes);
      tagBuilder4.AddCssClass(htmlHelper.ViewData.ModelState.IsValid ? HtmlHelper.ValidationSummaryValidCssClassName : HtmlHelper.ValidationSummaryCssClassName);
      tagBuilder4.InnerHtml = str + tagBuilder2.ToString(TagRenderMode.Normal);
      if (formContextForClientValidation != null)
      {
        if (htmlHelper.ViewContext.UnobtrusiveJavaScriptEnabled)
        {
          if (!excludePropertyErrors)
          {
            tagBuilder4.MergeAttribute("data-valmsg-summary", "true");
          }
        }
        else
        {
          tagBuilder4.GenerateId("validationSummary");
          formContextForClientValidation.ValidationSummaryId = tagBuilder4.Attributes["id"];
          formContextForClientValidation.ReplaceValidationSummary = !excludePropertyErrors;
        }
      }

      return new MvcHtmlString(tagBuilder4.ToString(TagRenderMode.Normal));
    }

    private static FormContext GetFormContextForClientValidation(ViewContext context)
    {
      return !context.ClientValidationEnabled ? null : context.FormContext;
    }

    private static string GetUserErrorMessageOrDefault(ModelError error)
    {
      return !string.IsNullOrEmpty(error.ErrorMessage) ? error.ErrorMessage : null;
    }
  }