HtmlPartialExtensions.cs

public static MvcHtmlString PartialFor<TModel, TProperty>(
    this HtmlHelper<TModel> helper,
    System.Linq.Expressions.Expression<Func<TModel, TProperty>> expression,
    string partialViewName,
    Dictionary<string, object> additionalViewData = null
    )
{
    // Get model value from expression
    string name = ExpressionHelper.GetExpressionText(expression);

    var modelMetadata = ModelMetadata.FromLambdaExpression(expression, helper.ViewData);
    var model = modelMetadata.Model;

    Type modelType = modelMetadata.ModelType;

    // Apply TemplateInfo to have hierarchical input naming convention
    var viewData = new ViewDataDictionary(helper.ViewData)
    {
        TemplateInfo = new TemplateInfo
        {
            HtmlFieldPrefix = string.IsNullOrWhiteSpace(helper.ViewData.TemplateInfo.HtmlFieldPrefix)
                ? name
                : helper.ViewData.TemplateInfo.HtmlFieldPrefix + "." + name
        }
    };

    // Copy additional key/values to the ViewData...
    additionalViewData = additionalViewData ?? new Dictionary<string, object>();

    foreach (var d in additionalViewData)
    {
        viewData[d.Key] = d.Value;
    }

    // Clear the parent model from the partial view's ViewDataDictionary
    viewData.Model = null;

    // Get PartialView MvcHtmlString...by passing in the new (sub-) model
    return helper.Partial(partialViewName, model, viewData);
}