如何使用的ToString SelectListItem实体框架？ [英] How to use ToString SelectListItem with Entity Framework?

问题描述

我有一些code如下图所示。我得到了运行时异常说道：

  

类型的异常的System.NotSupportedException发生在
  System.Data.Entity.dll但在用户code结果没有处理
      其他信息：LINQ实体无法识别方法'System.String的ToString（）'方法，和这种方法不能
  翻译成一家商店前pression。

由异常的原因VALUE = sup.SupplierID.ToString（）和VALUE = cat.CategoryID.ToString（）这似乎实体框架不能产生与的ToString（）但 SelectListItem 我必须给它一个字符串。

谁能提供简单的修复方案将是巨大的AP preciate。

---

 命名空间MvcApplication1.Models
{
    公共类ProductEditViewModel：产品
    {
        公共IEnumerable的＆LT; SelectListItem＆GT; SupplierDropDownItems {搞定;组; }
        公共IEnumerable的＆LT; SelectListItem＆GT; CategorieDropDownitems {搞定;组; }
    }
}
[HTTPGET]
公众的ActionResult ProductEdit（的Int32产品编号）
{
    VAR罗斯文=新NorthwindEntities（）;    变种Q =从northwind.Products p
            其中，p.ProductID ==产品编号
            选择新ProductEditViewModel
            {
                的ProductID = p.ProductID，
                产品名称= p.ProductName，
                单价= p.UnitPrice，
                SupplierDropDownItems =从燮在northwind.Suppliers选择新SelectListItem的{text = sup.CompanyName，值= sup.SupplierID.ToString（），选择= p.Supplier.SupplierID == p.SupplierID}，
                从猫CategorieDropDownitems =在northwind.Categories选择新SelectListItem的{text = cat.CategoryName，值= cat.CategoryID.ToString（），选择= p.Category.CategoryID == p.CategoryID}，
                停产= p.Discontinued
            };    返回查看（q.SingleOrDefault（））;
}
＆LT; D​​IV CLASS =表单组＆GT;
    @ Html.LabelFor（型号=＆GT; model.SupplierID，供应商ID，htmlAttributes：新{@class =控制标签COL-MD-2}）
    ＆LT; D​​IV CLASS =COL-MD-10＆GT;
        @ Html.DropDownList（供应商ID，Model.SupplierDropDownItems，htmlAttributes：新{@class =表格控}）
        @ Html.ValidationMessageFor（型号=＆GT; model.SupplierID，新{@class =TEXT-危险}）
    ＆LT; / DIV＆GT;
＆LT; / DIV＆GT;
 

解决方案

使用<一个href=\"http://msdn.microsoft.com/en-us/library/system.data.objects.sqlclient.sqlfunctions.stringconvert(v=vs.110).aspx\"相对=nofollow> SqlFunctions.StringConvert 这样的：

 值= SqlFunctions.StringConvert（（双）cat.CategoryID）
 

另一种选择是枚举内存中的所有记录，然后使用的ToString 这样的：

 从northwind.Suppliers.AsEnumerable SUP（）
选择新SelectListItem的{text = sup.CompanyName，值= sup.SupplierID.ToString（），选择= p.Supplier.SupplierID == p.SupplierID}，
 

AsEnumerable 会从数据库读取的所有记录，在内存中加载它们，所以你的电话的ToString 会不要求转换到SQL。

I have some code like below. I got run time exception says:

"An exception of type 'System.NotSupportedException' occurred in System.Data.Entity.dll but was not handled in user code
Additional information: LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression."

The exception cause by "Value = sup.SupplierID.ToString()" and "Value = cat.CategoryID.ToString()" It seems entity framework can not generate expression tree with ToString() but for SelectListItem I have to give it a string.

Who can provide simple fix solution will be great appreciate.

---

namespace MvcApplication1.Models
{
    public class ProductEditViewModel : Product
    {
        public IEnumerable<SelectListItem> SupplierDropDownItems { get; set; }
        public IEnumerable<SelectListItem> CategorieDropDownitems { get; set; }
    }
}


[HttpGet]
public ActionResult ProductEdit(Int32 ProductId)
{
    var northwind = new NorthwindEntities();

    var q = from p in northwind.Products
            where p.ProductID == ProductId
            select new ProductEditViewModel
            {
                ProductID = p.ProductID,
                ProductName = p.ProductName,
                UnitPrice = p.UnitPrice,
                SupplierDropDownItems = from sup in northwind.Suppliers select new SelectListItem { Text = sup.CompanyName, Value = sup.SupplierID.ToString(), Selected = p.Supplier.SupplierID == p.SupplierID },
                CategorieDropDownitems = from cat in northwind.Categories select new SelectListItem { Text = cat.CategoryName, Value = cat.CategoryID.ToString(), Selected = p.Category.CategoryID == p.CategoryID },
                Discontinued = p.Discontinued
            };

    return View(q.SingleOrDefault());
}


<div class="form-group">
    @Html.LabelFor(model => model.SupplierID, "SupplierID", htmlAttributes: new { @class = "control-label col-md-2" })
    <div class="col-md-10">
        @Html.DropDownList("SupplierID", Model.SupplierDropDownItems, htmlAttributes: new { @class = "form-control" })
        @Html.ValidationMessageFor(model => model.SupplierID, "", new { @class = "text-danger" })
    </div>
</div>

解决方案

Use SqlFunctions.StringConvert like:

Value = SqlFunctions.StringConvert((double)cat.CategoryID)

The other option is to enumerate all records in memory and then use ToString like:

from sup in northwind.Suppliers.AsEnumerable()
select new SelectListItem { Text = sup.CompanyName, Value = sup.SupplierID.ToString(), Selected = p.Supplier.SupplierID == p.SupplierID },

AsEnumerable would fetch all the records from the database and load them up in memory, so your call ToString would not be required to translate to SQL.

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