如何:显式加载 POCO 实体(实体框架)

本主题中的示例演示如何使用 ObjectContextLoadProperty 方法显式加载相关对象。 有关使用延迟加载访问相关对象的示例,请参见如何:使用延迟加载来加载相关对象(实体框架)LoadProperty 方法可用于 POCO 实体和从 EntityObject 派生的实体。

本主题中的示例使用在如何:定义 POCO 实体(实体框架)中定义的 POCO 数据类、在如何:定义自定义对象上下文(实体框架)中创建的 AdventureWorksEntities 类(派生自 ObjectContext)以及在如何:自定义建模和映射文件以使用自定义对象(实体框架)中定义的基于 AdventureWorks 的数据模型。

示例

此示例返回前五个 Order 对象并调用 LoadProperty 方法以显式加载每个 Order 的相关 LineItem 对象。

Using context As New POCOAdventureWorksEntities()
    Try
        ' Disable lazy loading. 
        context.ContextOptions.LazyLoadingEnabled = False

        ' Get the first five orders. 
        For Each order As Order In context.Orders.Take(5)
            ' Because LazyLoadingEnabled is set to false, 
            ' we need to explicitly load the related line items for the order. 
            context.LoadProperty(order, "LineItems")

            Console.WriteLine(String.Format("PO Number: {0}", order.ExtendedInfo.PurchaseOrderNumber))
            Console.WriteLine(String.Format("Order Date: {0}", order.OrderDate.ToString()))
            Console.WriteLine("Order items:")
            For Each item As LineItem In order.LineItems
                Console.WriteLine(String.Format("Product: {0} " & "Quantity: {1}", item.ProductID, item.OrderQty))
            Next
        Next
    Catch ex As InvalidOperationException
        Console.WriteLine(ex.Message)
    Catch ex As Exception
        Console.WriteLine(ex.Message)
    End Try
End Using
using (POCOAdventureWorksEntities context =
        new POCOAdventureWorksEntities())
{
    try
    {
        // Disable lazy loading.
        context.ContextOptions.LazyLoadingEnabled = false;

        // Get the first five orders.
        foreach (Order order in context.Orders.Take(5))
        {
            // Because LazyLoadingEnabled is set to false,
            // we need to explicitly load the related line items for the order.
            context.LoadProperty(order, "LineItems");

            Console.WriteLine(String.Format("PO Number: {0}",
                order.ExtendedInfo.PurchaseOrderNumber));
            Console.WriteLine(String.Format("Order Date: {0}",
                order.OrderDate.ToString()));
            Console.WriteLine("Order items:");
            foreach (LineItem item in order.LineItems)
            {
                Console.WriteLine(String.Format("Product: {0} "
                    + "Quantity: {1}", item.ProductID,
                    item.OrderQty));
            }
        }
    }
    catch (InvalidOperationException ex)
    {
        Console.WriteLine(ex.Message);
    }
    catch (Exception ex)
    {
        Console.WriteLine(ex.Message);
    }
}

另请参见

概念

加载相关 POCO 实体(实体框架)
自定义对象(实体框架)
使用 POCO 实体(实体框架)