Passing list of values to View as part of model from partial class

Question

User1047454725 posted

I have an auto-generated class file produced as part of the Entity model. So I'm creating a partial class to keep my customizations there in order to keep from getting over-written. My purpose is to keep things like static drop-down value lists as part of the class file in order to consume them in the View.

Auto-gen class:

namespace CMECFDictionaryChanges.Models
{
    using System;
    using System.Collections.Generic;
    
    public partial class Request
    {
        public Request()
        {
            this.AdminActions = new HashSet<AdminAction>();
        }
    
        public int Id { get; set; }
        public string RequestorName { get; set; }
        public System.DateTime RequestedOn { get; set; }
...

Custom Partial Class:

using System.Collections.Generic;

namespace CMECFDictionaryChanges.Models
{
    public partial class Request
    {
        private List<string> _changeTypeValues = new List<string> { "a", "b", "c" };
        
        public IEnumerable<string> ChangeTypeValues
        {
            get
            {
                return _changeTypeValues;
            } 
        }
    }
}

The View:

@model CMECFDictionaryChanges.Models.Request

@{
    Layout = "~/Views/Shared/_Layout.cshtml";
}
...

@using (Html.BeginForm("CreateRequest", "User", FormMethod.Post))
{
...
    <div class="row">
        <div class="col-xs-12 col-md-2">
            @Html.LabelFor(model => model.ChangeType, null, new { @class = "font-weight-bold" })
        </div>
        <div class="col-xs-12 col-md-4">
            @Html.DropDownListFor(model => model.ChangeType, new SelectList(Model.ChangeTypeValues), "Select", new { @class = "form-control" })
        </div>
    </div>
}

The problem is that the List defined in the partial class never gets initialized and as a result, you get the Object reference not set to an instance of an object error for the SelectList constructor. I'd rather avoid calling a method from the constructor of the auto-gen file to initialize the list because any customization in that file would be over-written.

Is there a better way of achieving this? Thanks in advance.

 

Answer 1

User475983607 posted

The partials are in two different namespaces and therefore two different types.   Just create a static class if the list is actually static.  This makes the list available throughout the project.

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/static-classes-and-static-class-members

Namespace reference

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/namespaces/

Answer 2

User1120430333 posted

I have an auto-generated class file produced as part of the Entity model. So I'm creating a partial class to keep my customizations there in order to keep from getting over-written. My purpose is to keep things like static drop-down value lists as part of the class file in order to consume them in the View.

Myself, I never mix/confuse the persistence model as being a view model. I use a view model for the view.

https://blog.sapiensworks.com/post/2012/04/07/Just-Stop-It!-The-Domain-Model-Is-Not-The-Persistence-Model.aspx

https://www.tutlane.com/tutorial/aspnet-mvc/how-to-use-viewmodel-in-asp-net-mvc-with-example

Answer 3

User-271186128 posted

Hi kj27,

Have you solved this error?

The problem is that the List defined in the partial class never gets initialized and as a result, you get the Object reference not set to an instance of an object error for the SelectList constructor. I'd rather avoid calling a method from the constructor of the auto-gen file to initialize the list because any customization in that file would be over-written.

As for this issue, you could create a Request instance in the CreateRequest HttpGet action method.

Code as below:

        [HttpGet]
        public ActionResult CreateRequest()
        {
            Request request = new Request();
            return View(request);
        }

Best regards,
Dillion