出现错误“用户代码未处理 NotsupportedException”

Question

我有一个表，

  product(table name)
        product_id
        product_name 
        product_image
        product_price
        product_description
        category_id

  category(table name )
        category_id
        category_name 
        category_description

我有一个名为 categoryCombobox 的组合框，并将其命名为 productgridview 的网格视图，

我试图根据组合框中的选择来填充数据网格..就像这样。 ...

       private viod form_load(object sender, EventArgs e)
       {
        var products = from prods in abc.products
                       select new
                       {
                           prods.product_Id,
                           productname =  prods.product_Name,
                           productimage = prods.product_Image,
                           productprice = prods.product_Price,
                           productdescription = prods.product_Description

                       };
        productbindingsource.DataSource = products;
        productgridview.DataSource = productbindingsource;
        productgridview.Columns[0].Visible = false;
       }
       private void categoryCombobox_SelectedIndexChanged(object sender, EventArgs e)
       {

          // is this query correct 
          var categoryid = from productcategories in abc.categories
                         where productcategories.category_Name.Equals(categoryCombobox.Text)
                         select productcategories.category_Id;

          var produc = from pros in abc.products
                       where pros.Category_Id.Equals(categoryid)
                       select new
                       {
                           productname = pros.product_Name,
                           productimage = pros.product_Image,
                           productprice = pros.product_Price,
                           productdescription = pros.product_Description                                   

                       };
        productbindingsource.DataSource = produc;
        productgridview.DataSource = productbindingsource;
        productgridview.Columns[0].Visible = false;

    }      

出现这样的错误......

错误：在这一行productbindingsource.DataSource = produc;

不支持用户代码未处理异常
无法比较“System.Linq.IQueryable`1”类型的元素。 仅原始类型（例如 Int32、String 和 Guid）和实体 支持类型。

Answer 1

var categoryid = from productcategories in abc.categories
                 where productcategories.
                           category_Name.Equals(categoryCombobox.Text)
                 select productcategories.category_Id;

调试时将鼠标悬停在 var 上。您将看到它不是您期望的 id，而是一个 IEnumerable。您要做的是

// .First() trips the query and returns a single category_Id
var id = (from productcategories in abc.categories
         where productcategories.
                   category_Name.Equals(categoryCombobox.Text)
         select productcategories.category_Id).First();

var produc = from pros in abc.products
               where pros.Category_Id.Equals(id)
               select new
               {
                   productname = pros.product_Name,
                   productimage = pros.product_Image,
                   productprice = pros.product_Price,
                   productdescription = pros.product_Description                                   
               };

注意 ids.First()，它从初始查询中获取第一个结果。

Answer 2

您试图将 int 字段与可枚举集进行比较。

如果categoryID 查询仅返回单个值，请尝试以下操作：

var produc = from pros in abc.products
                   where pros.Category_Id.Equals(categoryid.Single())
                   select new
                   {
                       productname = pros.product_Name,
                       productimage = pros.product_Image,
                       productprice = pros.product_Price,
                       productdescription = pros.product_Description                                   

                   };

如果它应该返回 Id 列表，则您将需要编写带有联接的单个查询。我认为根据名称 categoryId

编辑它应该是单一的 - 可能不是 100% 语法正确

var produc = from pros in abc.products
    join cats in abc.categories on cats.category_id equals pros.Category_Id
    where cats.category_Name.Equals(categoryCombobox.Text)
    select new
    {
        productname = pros.product_Name,
        productimage = pros.product_Image,
        productprice = pros.product_Price,
        productdescription = pros.product_Description                                   
    };