無法在WCF Web數據服務中訪問數據庫

我無法訪問數據庫（在實體框架中）。每次嘗試時，都會顯示此錯誤：無法在WCF Web數據服務中訪問數據庫

處理此請求時發生錯誤。

我沒有得到正確的錯誤信息，這會幫助我更好地理解。

另外，我可以在WCF中使用SqlConnection而不是實體框架嗎？

using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Data; 
using System.Drawing; 
using System.Linq; 
using System.Text; 
using System.Windows.Forms; 
using AutoSaleClient.ServiceReference; 
using System.Data.Services.Client; 
namespace AutoSaleClient 
{ 
    public partial class LoginWindow : Form 
    { 
     private System.Windows.Forms.BindingSource booksBindingSource; 
     public LoginWindow() 
     { 
      InitializeComponent(); 
     } 
       private void button1_Click(object sender, EventArgs e) 
     { 
      DataServiceContext autoSaleContext = new DataServiceContext(new Uri("http://localhost:3371/AutoSaleDataService.svc/")); 
      AutoSaleSystemDbEntities db = new AutoSaleSystemDbEntities(new Uri("http://localhost:3371/AutoSaleDataService.svc/")); 
      // db.IgnoreMissingProperties = true; 

      // _dataContext = new JustBooks_P000002Entities(Classes.Utilities.DataServiceAddress); 

      this.booksBindingSource.DataSource = new DataServiceCollection<Car>(db.Cars.Expand("CarId")); 
      // var q = from r in db.RigisterdUsers select r; 
      Console.WriteLine("this is test ",this.booksBindingSource.ToString()); 

      new MainForm().Show(); 
      this.Hide(); 
     } 

     private void radioButton1_CheckedChanged(object sender, EventArgs e) 
     { 

     } 
    } 
}

error