ASP.Net MVC dropdown selected value while editing a record

I have been dealing with the dropdownlist which i am able to populate but if i come back to edit a record, the dropdown is not showing the selected value.....

In controller,

        var datacontext = new ServicesDataContext();
        var serviceToUpdate = datacontext.Mapings.First(m => m.CustomerServiceMappingID == id);
        var qw = (from m in datacontext.Mapings
                  where id == m.CustomerServiceMappingID
                  select m.CustomerID).First();
        ViewData["CustomerID"] = qw;

        var a = datacontext.services.Select(arg => arg.ServiceID).ToList();
        ViewData["ServiceID"] = new SelectList(a,serviceToUpdate.ServiceID);

In view:

    @Html.DropDownListFor(model => model.ServiceID, ViewData["ServiceID"] as SelectList)

serviceToUpdate,ServiceID has the right value but 开发者_如何学运维somehow when I try to edit the selected value is not returned... instead the first value of dropdown is only returned...

However, I turned one more thing where in controller, I did something like this,

      ViewData["ServiceID"] = new  SelectList(a,"ServiceID","ServiceID");

Here, I get an error saying

           'System.Int32' does not contain a property with the name 'ServiceID'.

What happens when you change

ViewData["ServiceID"] = new  SelectList(a,"ServiceID","ServiceID");

to

ViewData["SelectListServiceID"] = new  SelectList(a,"ServiceID","ServiceID");

and in your view:

@Html.DropDownListFor(model => model.ServiceID, ViewData["SelectListServiceID"] as SelectList)

I have seen this before and it has to do with automatic binding. You probably have a property named ServiceID in your model.