MVC 3 CheckboxList and me...Part 3. Try specifying the type arguments explicitly - asp.net-mvc

Question

This IS fun.
Ok, I have the following model(s):
public class My : BusinessCategory
{
[Display(Name = "What types of energy do you use?")]
public List<MyTypes> MyTypeList { get; set; }
public bool? FirstOption { get; set; }
public bool? SecondOption{ get; set; }
public bool? ThirdOption{ get; set; }
public bool? FourthOption { get; set; }
}
Where MyTypes:
public class MyTypes
{
public int MyTypeId { get; set; }
public string MyTypeName { get; set; }
public bool? MyTypeValue { get; set; }
}
My controller is as follows:
public ActionResult My(Guid id)
{
try
{
var model = Model(id);
SetMyTypeList(model.My);
ViewBag.MyTypeMe = new MultiSelectList(model.My.MyTypeList, "MyTypeValue", "MyTypeName");
return View(model.My);
}
catch (Exception ex)
{
ExceptionHelper.WriteLog(ex);
return RedirectToAction("Error");
}
}
private void SetMyTypeList(My model)
{
model.MyTypeList = new List<MyTypes>();
model.MyTypeList.Add(new MyTypes { MyTypeId = 1, MyTypeName = GetName.GetDisplayName(model, m => m.FirstOption), MyTypeValue = model.FirstOption });
model.MyTypeList.Add(new MyTypes { MyTypeId = 2, MyTypeName = GetName.GetDisplayName(model, m => m.SecondOption), MyTypeValue = model.SecondOption});
model.MyTypeList.Add(new MyTypes { MyTypeId = 3, MyTypeName = GetName.GetDisplayName(model, m => m.ThirdOption), MyTypeValue = model.ThirdOption});
model.MyTypeList.Add(new MyTypes { MyTypeId = 4, MyTypeName = GetName.GetDisplayName(model, m => m.FourthOption), MyTypeValue = model.FourthOption });
}
public static string GetDisplayName<TModel, TProperty>(TModel model, Expression<Func<TModel, TProperty>> expression)
{
return ModelMetadata.FromLambdaExpression<TModel, TProperty>(expression, new ViewDataDictionary<TModel>(model)).DisplayName;
}
And finally the view is as follows:
#model Valpak.Websites.HealthChecker.Models.My
#{
ViewBag.Title = "My";
}
<h2>
My</h2>
#using (Html.BeginForm())
{
#Html.ValidationSummary(true)
<fieldset>
<legend>My Management</legend>
<div style="text-align: left; padding-left: 47%;">
#Html.ListBoxFor(model => model.MyTypeList, ViewBag.MyTypeMe as MultiSelectList)
#Html.CheckBoxListFor(model => model.MyTypeList, ViewBag.EnergyTypeMe as MultiSelectList, Model.ReviewId)
</div>
<p>
<input type="submit" value="Continue" />
</p>
</fieldset>
}
<div>
#Html.ActionLink("Cancel and return to categories", "BusinessSummary", new { id = Model.ReviewId })
</div>
CheckboxListFor, if it was working, would use the following extension:
public static class HtmlHelper
{
//Extension
public static MvcHtmlString CheckBoxListFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty[]>> expression, MultiSelectList multiSelectList, object htmlAttributes = null)
{
//Derive property name for checkbox name
MemberExpression body = expression.Body as MemberExpression;
string propertyName = body.Member.Name;
//Get currently select values from the ViewData model
TProperty[] list = expression.Compile().Invoke(htmlHelper.ViewData.Model);
//Convert selected value list to a List<string> for easy manipulation
List<string> selectedValues = new List<string>();
if (list != null)
{
selectedValues = new List<TProperty>(list).ConvertAll<string>(delegate(TProperty i) { return i.ToString(); });
}
//Create div
TagBuilder divTag = new TagBuilder("div");
divTag.MergeAttributes(new RouteValueDictionary(htmlAttributes), true);
//Add checkboxes
foreach (SelectListItem item in multiSelectList)
{
divTag.InnerHtml += String.Format("<div><input type=\"checkbox\" name=\"{0}\" id=\"{0}_{1}\" " +
"value=\"{1}\" {2} /><label for=\"{0}_{1}\">{3}</label></div>",
propertyName,
item.Value,
selectedValues.Contains(item.Value) ? "checked=\"checked\"" : "",
item.Text);
}
return MvcHtmlString.Create(divTag.ToString());
}
}
Can someone you explain in very simplistic terms (I’m a bit dense) why I can use the ListBoxFor example but this dies and gives me the following error when I use the checkbox?
CS0411: The type arguments for method 'Extensions.HtmlHelper.CheckBoxListFor<TModel,TProperty>(System.Web.Mvc.HtmlHelper<TModel>, System.Linq.Expressions.Expression<System.Func<TModel,TProperty[]>>, System.Web.Mvc.MultiSelectList, System.Guid, object)' cannot be inferred from the usage. Try specifying the type arguments explicitly.
Can anyone offer any sort of work around as I’d quite like to use my :’(
As always, apologies for my ignorance.

Answer 1

In the signature of the extension method you have the following second argument:
Expression<Func<TModel, TProperty[]>> expression,
This basically means that the expression must return an array of TProperty => TProperty[]
whereas in your view model you have a List<T>:
public List<MyTypes> EnergyTypeList { get; set; }
and inside your view you are using:
model => model.EnergyTypeList
Your code doesn't work because List<EnergyTypeList> is not the same thing as EnergyTypeList[].
So you have different possibilities. Either change the type in your view model to match the one in your helper or use change your helper to use a List or even better an IEnumerable<TProperty>. This way the extension method will work even with arrays.