ObjectQuery无效参数

Question

我很难从db_Entities模型中过滤数据。我有一个绑定到comboBox1的表，并且我正在尝试使用comboBox1控件中的commitChanged事件筛选数据。我收到两个错误。1是：'System.Data.Entity.Core.Objects.ObjectQuery.ObjectQuery(string，System.Data.Entity.Core.Objects.ObjectContext)‘的最佳重载方法匹配包含一些无效参数。

第二个问题是:无法从'SmallStore.db_Entities‘转换为'System.Data.Entity.Core.Objects.ObjectContext'.为了节省空间，我省略了目录。以下是代码

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Data.Entity.Core.Objects;
    using System.Data.Entity.Core.Query;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Threading.Tasks;
    using System.Windows.Forms;


    namespace SmallStore
    {
         public partial class ViewProducts : Form
         {                
           private db_Entities db = new db_Entities();

    public ViewProducts()
    {

        InitializeComponent();

        dataGridView1.DataSource = db.Products.ToList();

        dataGridView1.Columns["ProductType"].Visible = false;
        dataGridView1.Columns["TransactionItems"].Visible = false;
        dataGridView1.Columns["Product_Type"].Visible = false;

        comboBox1.DataSource = db.Product_Type.ToList();
        comboBox1.ValueMember = "Description";
        comboBox1.DisplayMember = "Product_Type";

    }

    private void changeCommit(object sender, EventArgs e)
    {
        /* filter products */
        ObjectQuery<Product> filteredProducts = new ObjectQuery<Product>("SELECT VALUE item FROM Products AS item WHERE item.ProductType = " + comboBox1.SelectedValue, db);

    }
}

}

Answer 1

试试这个：

var querystring = "SELECT VALUE item FROM Products AS item WHERE item.ProductType = " + comboBox1.SelectedValue;  

Debugger.Break();  //Ensure querystring is what you want here

 var products = db.Database.SqlQuery<Product>(querystring).ToList();