SharePoint Foundation 2010 托管客户端对象模型概述（九） 访问大型列表

SharePoint 开发指南指示，您不应尝试在单个查询中检索 2000 个以上的项目。如果您的应用程序中可能会出现这种情况，可以考虑在 CAML 查询中使用 RowLimit 元素，以限制客户端对象模型可以为应用程序检索的数据量。有时您必须访问可能包含 2000 个以上项目的列表中的所有项目。如果必须如此，则最好进行分页，一次查看 2000 个项目。本节介绍了使用 ListItemCollectionPosition 属性进行分页的方法。

C#
using System;
using System.Linq;
using Microsoft.SharePoint.Client;

class Program
{
    static void Main()
    {
        ClientContext clientContext =
            new ClientContext("http://intranet.contoso.com");

        List list = clientContext.Web.Lists
            .GetByTitle("Client API Test List");

        // First, add 20 items to Client API Test List so that there are
        // enough records to show paging.
        ListItemCreationInformation itemCreateInfo =
            new ListItemCreationInformation();
        for (int i = 0; i < 20; i++)
        {
            ListItem listItem = list.AddItem(itemCreateInfo);
            listItem["Title"] = String.Format("New Item #{0}", i);
            listItem["Category"] = "Development";
            listItem["Estimate"] = i;
            listItem.Update();
        }
        clientContext.ExecuteQuery();

        // This example shows paging through the list ten items at a time.
        // In a real-world scenario, you would want to limit a page to
        // 2000 items.
        ListItemCollectionPosition itemPosition = null;
        while (true)
        {
            CamlQuery camlQuery = new CamlQuery();
            camlQuery.ListItemCollectionPosition = itemPosition;
            camlQuery.ViewXml =
                @"<View>
                    <ViewFields>
                      <FieldRef Name='Title'/>
                      <FieldRef Name='Category'/>
                      <FieldRef Name='Estimate'/>
                    </ViewFields>
                    <RowLimit>10</RowLimit>
                  </View>";
            ListItemCollection listItems = list.GetItems(camlQuery);
            clientContext.Load(listItems);
            clientContext.ExecuteQuery();
            itemPosition = listItems.ListItemCollectionPosition;
            foreach (ListItem listItem in listItems)
                Console.WriteLine("  Item Title: {0}", listItem["Title"]);
            if (itemPosition == null)
                break;
            Console.WriteLine(itemPosition.PagingInfo);
            Console.WriteLine();
        }
    }
}
该示例将生成以下输出：

  Item Title: Write specs for user interface.
  Item Title: Develop proof-of-concept.
  Item Title: Write test plan for user interface.
  Item Title: Validate SharePoint interaction.
  Item Title: Develop user interface.
  Item Title: New Item #0
  Item Title: New Item #1
  Item Title: New Item #2
  Item Title: New Item #3
  Item Title: New Item #4
Paged=TRUE&p_ID=10

  Item Title: New Item #5
  Item Title: New Item #6
  Item Title: New Item #7
  Item Title: New Item #8
  Item Title: New Item #9
  Item Title: New Item #10
  Item Title: New Item #11
  Item Title: New Item #12
  Item Title: New Item #13
  Item Title: New Item #14
Paged=TRUE&p_ID=20

  Item Title: New Item #15
  Item Title: New Item #16
  Item Title: New Item #17
  Item Title: New Item #18
  Item Title: New Item #19

异步处理
如果您要构建必须附加到可能不可用的 SharePoint 网站的应用程序，或者您必须定期调用可能要花很长时间的查询，则应考虑使用异步处理。这样，当查询在单独的线程中执行时，您的应用程序能够继续响应用户。在主线程中，您可以设置一个计时器。如果查询所花时间长于所需阈值，该计时器可通知您，以便您可以告诉用户查询状态，并在查询最终完成时显示结果。

客户端对象模型的 JavaScript 版本和 Silverlight 版本（在修改用户界面时）都使用异步处理。SharePoint 2010 SDK 中的主题Data Retrieval Overview包含有关如何通过 JavaScript 和 Silverlight 使用异步处理的示例。

在构建基于 .NET Framework 的传统应用程序（如 Windows 窗体或 WPF 应用程序）时，可能需要使用异步处理。以下示例使用 BeginInvoke 方法异步执行查询。请注意，该代码会将一个语句式 lambda 表达式传递给 BeginInvoke 方法，这样便可方便地创建此模式，因为语句式 lambda 表达式可以引用该表达式所在方法中的自动变量。您可以看到语句式 lambda 表达式可以访问 clientContext 变量和 newListCollection 变量。Microsoft Visual C# 闭包使该语言能够按您预期的方式工作。

C#
using System;
using System.Collections.Generic;
using Microsoft.SharePoint.Client;

class Program
{
    static void Main(string[] args)
    {
        AsynchronousAccess asynchronousAccess = new AsynchronousAccess();
        asynchronousAccess.Run();
        Console.WriteLine("Before exiting Main");
        Console.WriteLine();
        Console.WriteLine("In a real application, the application can");
        Console.WriteLine("continue to be responsive to the user.");
        Console.WriteLine();
        Console.ReadKey();
    }
}

class AsynchronousAccess
{
    delegate void AsynchronousDelegate();

    public void Run()
    {
        Console.WriteLine("About to start a query that will take a long time.");
        Console.WriteLine();
        ClientContext clientContext =
            new ClientContext("http://intranet.contoso.com");
        ListCollection lists = clientContext.Web.Lists;
        IEnumerable<List> newListCollection = clientContext.LoadQuery(
            lists.Include(
                list => list.Title));
        AsynchronousDelegate executeQueryAsynchronously =
            new AsynchronousDelegate(clientContext.ExecuteQuery);
        executeQueryAsynchronously.BeginInvoke(
            arg =>
            {
                clientContext.ExecuteQuery();
                Console.WriteLine("Long running query completed.");
                foreach (List list in newListCollection)
                    Console.WriteLine("Title: {0}", list.Title);
            }, null);
    }
}
该示例将生成以下输出。

About to start a query that will take a long time.

Before exiting Main

In a real application, the application can
continue to be responsive to the user.

Long running query completed.
Title: Announcements
Title: Cache Profiles
Title: Calendar
Title: Client API Test List
Title: Content and Structure Reports
Title: Content type publishing error log
Title: Converted Forms
Title: Customized Reports
Title: Eric's ToDo List
Title: Eric's Wiki
Title: Form Templates
Title: Links