How to fix conversion error in MVC2 dropdownlist

问题

I tried simple dropdownlist in ASP.NET MVC2 but got error below from tryupdatemodel.

How to fix this, everthing looks correct ?

view:

   <%= Html.DropDownList("Post24DeliveryPlaces", Model.DeliveryPlace)%>

model:

public IEnumerable<SelectListItem> Post24DeliveryPlaces { get; set; }

public string DeliveryPlace { get; set; }

result:

System.InvalidOperationException: The parameter conversion from type 'System.String' to type 'System.Web.Mvc.SelectListItem' failed because no type converter can convert between these types.  

at System.Web.Mvc.ValueProviderResult.ConvertSimpleType(CultureInfo culture, Object value, Type destinationType)  

 at System.Web.Mvc.ValueProviderResult.UnwrapPossibleArrayType(CultureInfo culture, Object value, Type destinationType)  

at System.Web.Mvc.ValueProviderResult.ConvertTo(Type type, CultureInfo culture)   at System.Web.Mvc.DefaultModelBinder.ConvertProviderResult(ModelStateDictionary modelState, String modelStateKey, ValueProviderResult valueProviderResult, Type destinationType

回答1:

The problem is that your html input is being named Post24DeliveryPlaces and it's trying to bind a single string to that.

The Post data is looking like this:

POST
{
    Post24DeliveryPlaces = (string)
}

However, in your model, Post24DeliveryPlaces is an IEnumerable and it can't convert at string to SelectListItem.

Html.DropDownList takes the field name as the first parameter. Try changing to this, giving the list of delivery places as the second parameter.

Html.DropDownList("DeliveryPlace", Model.Post24DeliveryPlaces)

This will generate the Html

<select name="DeliveryPlace">
    <option value="..">....</option>
    ....
</select>

来源：https://stackoverflow.com/questions/9218011/how-to-fix-conversion-error-in-mvc2-dropdownlist