Question

I wrote a helper method to display enums from my model in my asp.net MVC application as drop down lists in my views.

Here is my code for that:

public static List<SelectListItem> CreateSelectItemList<TEnum>(object enumObj,
                                                            string defaultItemKey,
                                                            bool sortAlphabetically,
                                                            object firstValueOverride)
    where TEnum : struct
    {
        var values = (from v in (TEnum[])Enum.GetValues(typeof(TEnum))
                      select new
                      {
                          Id = Convert.ToInt32(v),
                          Name = ResourceHelpers.GetResourceValue(AppConstants.EnumResourceNamespace,
                                                                  typeof(TEnum).Name, v.ToString())
                      });


        return values.ToSelectList(e => e.Name,
                                               e => e.Id.ToString(),
                                               !string.IsNullOrEmpty(defaultItemKey) ? ResourceHelpers.GetResourceValue(AppConstants.EnumResourceNamespace, defaultItemKey) : string.Empty,
                                               enumObj,
                                               sortAlphabetically,
                                               firstValueOverride);

    }

This actually generates the select item list:

public static List<SelectListItem> ToSelectList<T>(
    this IEnumerable<T> enumerable,
    Func<T, string> text,
    Func<T, string> value,
    string defaultOption,
    object selectedVal,
    bool sortAlphabetically,
    object FirstValueOverride)
{

    int iSelectedVal = -1;

    if(selectedVal!=null)
    {
        try
        {
            iSelectedVal = Convert.ToInt32(selectedVal);
        }
        catch
        {
        }
    }

    var items = enumerable.Select(f => new SelectListItem()
    {
        Text = text(f),
        Value = value(f),
        Selected = (iSelectedVal > -1)? (iSelectedVal.ToString().Equals(value(f))) : false
    });

    #region Sortare alfabetica
    if (sortAlphabetically)
        items = items.OrderBy(t => t.Text);
    #endregion Sortare alfabetica

    var itemsList = items.ToList();

    Func<SelectListItem, bool> funct = null;
    string sValue = string.Empty;
    SelectListItem firstItem = null;
    SelectListItem overridenItem = null;
    int overridenIndex = 0;

    if (FirstValueOverride != null)
    {
        sValue = FirstValueOverride.ToString();

        funct = (t => t.Value == sValue);
        overridenItem = itemsList.SingleOrDefault(funct);
        overridenIndex = itemsList.IndexOf(overridenItem);

        if (overridenItem != null)
        {
            firstItem = itemsList.ElementAt(0);
            itemsList[0] = overridenItem;
            itemsList[overridenIndex] = firstItem;
        }
    }

    if(!string.IsNullOrEmpty(defaultOption))
        itemsList.Insert(0, new SelectListItem()
        {
            Text = defaultOption,
            Value = "-1"
        });

    return itemsList;
}

These is the method I call:

        public static MvcHtmlString EnumDropDownList<TEnum>(this HtmlHelper htmlHelper, 
                                                        object enumObj,
                                                        string name,
                                                        string defaultItemKey,
                                                        bool sortAlphabetically,
                                                        object firstValueOverride,
                                                        object htmlAttributes)
    where TEnum : struct
    {
        return htmlHelper.DropDownList(name,
                                        CreateSelectItemList<TEnum>(enumObj,
                                                                defaultItemKey,
                                                                sortAlphabetically,
                                                                firstValueOverride), 
                                         htmlAttributes);
    }

Now I am having the problem described here
When I call this helper method and the input’s name is the same as the property’s name the selected value doesn’t get selected.
The alternate solution described there doesn’t work for me. The only solution that works is changing the name and not using the model binding using FormCollection instead.
I don’t like this workaround because I can’t use validation any more using the ViewModel pattern and I have to write some extra code for every enum.
I tried writing a custom model binder to compensate for this somehow but none of the methods I can override there gets called when I start the action.

Is there any way to do this without using FormCollection?
Can I somehow intercept ASP.NET MVC when it tries to put the value into my input field and make it select the right value?

Thank you in advance.

Answer 1

I have a sollution now
Here is the code for the EnumDropDownList function the other functions are listed in the question:

public static MvcHtmlString EnumDropDownList(this HtmlHelper htmlHelper,
                                                   object enumObj,
                                                   string name,
                                                   string defaultItemKey,
                                                   bool sortAlphabetically,
                                                   object firstValueOverride,
                                                   object htmlAttributes)
{

    StringBuilder sbRezultat = new StringBuilder();

    int selectedIndex = 0;

    var selectItemList = new List<SelectListItem>();


    if (enumObj != null)
    {
        selectItemList = CreateSelectItemList(enumObj, defaultItemKey, true, null);

        var selectedItem = selectItemList.SingleOrDefault(item => item.Selected);
        if (selectedItem != null)
        {
            selectedIndex = selectItemList.IndexOf(selectedItem);

        }
    }

    var dict = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);


    TagBuilder tagBuilder = new TagBuilder("select");

    tagBuilder.MergeAttribute("name", name,true);

    bool bReadOnly = false;

    //special case for readonly
    if(dict.ContainsKey("readonly"))
    {
        //remove this tag it won't work the way mvc renders it anyway
        dict.Remove("readonly");
        bReadOnly = true;
    }

    //in case the style element is completed if the drop down is not readonly
    tagBuilder.MergeAttributes(dict, true);

    if (bReadOnly)
    {
        //add a small javascript to make it readonly and add the lightgrey style
        tagBuilder.MergeAttribute("onchange", "this.selectedIndex=" + selectedIndex + ";",true);
        tagBuilder.MergeAttribute("style", "background: lightgrey", true);
    }

    sbRezultat.Append(tagBuilder.ToString(TagRenderMode.StartTag));


    foreach (var option in selectItemList)
    {
        sbRezultat.Append(" <option value='");
        sbRezultat.Append(option.Value);
        sbRezultat.Append("' ");
        if (option.Selected)
            sbRezultat.Append("selected");
        sbRezultat.Append(" >");
        sbRezultat.Append(option.Text);


        sbRezultat.Append("</option>");
    }
    sbRezultat.Append("</select>");
    return new MvcHtmlString(sbRezultat.ToString());
}

I also wrote a generic function of type For (EnumDropDownFor):

public static MvcHtmlString EnumDropDownListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper,
    Expression<Func<TModel, TProperty>> expression,
                                string defaultItemKey,
                                bool sortAlphabetically,
                                object firstValueOverride,
                                object htmlAttributes)
    where TProperty : struct
{
    string inputName = GetInputName(expression);

    object selectedVal = null;
    try
    {
        selectedVal = htmlHelper.ViewData.Model == null
            ? default(TProperty)
            : expression.Compile()(htmlHelper.ViewData.Model);
    }
    catch//in caz ca e ceva null sau ceva de genu'
    {
    }

    return EnumDropDownList(htmlHelper,
                            selectedVal,
                            inputName,
                            defaultItemKey,
                            sortAlphabetically,
                            firstValueOverride,
                            htmlAttributes);
}

and some helper methods:

public static string GetInputName<TModel, TProperty>(Expression<Func<TModel, TProperty>> expression)
{
    if (expression.Body.NodeType == ExpressionType.Call)
    {
        MethodCallExpression methodCallExpression = (MethodCallExpression)expression.Body;
        string name = GetInputName(methodCallExpression);
        return name.Substring(expression.Parameters[0].Name.Length + 1);

    }
    return expression.Body.ToString().Substring(expression.Parameters[0].Name.Length + 1);
}

private static string GetInputName(MethodCallExpression expression)
{
    MethodCallExpression methodCallExpression = expression.Object as MethodCallExpression;
    if (methodCallExpression != null)
    {
        return GetInputName(methodCallExpression);
    }
    return expression.Object.ToString();
}