数据库中的数据集为空 [英] Dataset from database is empty

问题描述

我正在从我的数据库访问一个表。我在Visual Studio 2013中添加了表作为数据集，但是当我尝试使用它时，它会显示为空。

这是我要做的：

  IQueryable< NorthwindDataSet.OrdersRow> LookupOrdersForYear（int year）
 {
 using（var context = new NorthwindDataSet（））
 {
 var orders = 
 from context.Orders 
其中order.OrderDate！= null&&& order.OrderDate.Year> =年
选择订单; 
 return orders.ToList（）。AsQueryable（）; 
} 
} 
  

我发现 是空的，所以我添加了

  Console.WriteLine（context.Orders.Count）; 
  

这给我的输出为0.出了什么问题？

解决方案

我发现我需要用数据填充数据集，否则会保持为空。

SqlDataAdapter做了一个诀窍：

  SqlDataAdapter adapter = new SqlDataAdapter（）; 
 SqlCommand command = new SqlCommand（SELECT * FROM Orders）; 
 string connString = @Data Source = localhost\SQLEXPRESS; Initial Catalog = Northwind; Integrated Security = True; 
 SqlConnection conn = new SqlConnection（connString）; 
 
 adapter.SelectCommand = command; 
 adapter.SelectCommand.Connection = conn; 
 adapter.Fill（context.Orders）; 
  

---

SqlDataAdapter.Fill有几种重载方法，其中一个采用数据集，另一个采用数据。当我第一次使用SqlDataAdapter时，我使用了一个数据集

  adapter.Fill（context）; 
  

其中还给了我一个空的context.Orders。正确的一个需要一个数据表：

  adapter.Fill（context.Orders）; 
  

这个工作并按预期返回日期。

I'm accessing a table from my database. I have added the tables as a dataset in Visual Studio 2013, but when I try to use it, it comes out empty.

This is what I'm trying to do:

IQueryable<NorthwindDataSet.OrdersRow> LookupOrdersForYear(int year)
{
    using (var context = new NorthwindDataSet())
    {
        var orders =
            from order in context.Orders
            where order.OrderDate != null && order.OrderDate.Year >= year
            select order;
        return orders.ToList().AsQueryable();
    }
}

I found out that orders was empty, so I added

Console.WriteLine(context.Orders.Count);

which gave me an output of 0. What went wrong?

解决方案

I found out that I needed to fill my dataset with data, or it would stay empty.

A SqlDataAdapter did the trick:

SqlDataAdapter adapter = new SqlDataAdapter();
SqlCommand command = new SqlCommand("SELECT * FROM Orders");
string connString = @"Data Source=localhost\SQLEXPRESS;Initial Catalog=Northwind;Integrated Security=True";
SqlConnection conn = new SqlConnection(connString);

adapter.SelectCommand = command;
adapter.SelectCommand.Connection = conn;
adapter.Fill(context.Orders);

---

There are several overloaded methods for SqlDataAdapter.Fill, of which one takes a dataset and another takes a datatable. When I first used the SqlDataAdapter, I used the one that takes a dataset

adapter.Fill(context);

which still gave me an empty context.Orders. The correct one takes a datatable:

adapter.Fill(context.Orders);

This one worked and returned the dates as expected.

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