asp.net - LINQ to Entities 无法识别方法 'System.String ToString()' 方法，并且该方法无法转换为存储表达式

Question

这是我的 Controller 类

public class HomeController : Controller
{
    private rikuEntities rk = new rikuEntities();
    public ActionResult Index()
    {
        var db = new rikuEntities();
        IEnumerable<SelectListItem> items = db.emp.Select(c => new     
               SelectListItem
               {
                 Value = c.Id.ToString(), 
                 Text = c.name
               });
        ViewBag.CategoryID = items;
        return View();
    }
}

这是我的观点

@using (Html.BeginForm("viewToController", "Home"))
{
    @Html.ValidationSummary(true)
    <fieldset>
        <legend>emp</legend>

        <div class="editor-field">
            @Html.DropDownList("CategoryID", (IEnumerable<SelectListItem>) ViewBag.Categories)
        </div>
        <p>
            <input type="submit" value="Create" />
        </p>
    </fieldset>
}

每当我运行这个程序时，我都会收到此错误:

LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression." in the statement @Html.DropDownList("CategoryID", (IEnumerable) ViewBag.Categories). i am using entity framework mechanism for databse connection. please help me to find out the error...

Answer 1

我建议您使用 View 模型和强类型 View 而不是 ViewBag。因此，从定义 View 模型开始:

public class EmployeeViewModel
{
    public string CategoryId { get; set; }
    public IEnumerable<Employee> Categories { get; set; }
}

然后在 Controller 中填充此 View 模型:

public class HomeController : Controller
{
    public ActionResult Index()
    {
        var db = new rikuEntities();
        var model = new EmployeeViewModel
        {
            Categories = db.emp.ToArray() // <-- you probably want categories here
        };
        return View(model);
    }
}

在 View 中:

@model EmployeeViewModel
@using (Html.BeginForm("viewToController", "Home"))
{
    @Html.ValidationSummary(true)
    <fieldset>
        <legend>emp</legend>

        <div class="editor-field">
            @Html.DropDownListFor(
                x => x.CategoryId,
                new SelectList(Model.Categories, "Id", "name")
            )
        </div>
        <p>
            <input type="submit" value="Create" />
        </p>
    </fieldset>
}