我正在构建一个标准的odata客户端,使用:Microsoft.Data.Services.Client.PortableWindows 8VS2013
我已经授权给项目(TMALiveData)添加了一个服务引用。现在我想检索数据。我正在使用下面的代码,但是当我这样做时,我在最后的循环中得到一个空指针引用。
代码是:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data.Services.Client;
namespace testLSconWithMSv2
{
public class testLSCon
{
static string mResult;
public static string result { get { return mResult; } }
public static void testREADLiveConnection()
{
Uri tmaLiveDataRoot = new Uri("https://xxx.azurewebsites.net/xxx.svc/");
TMLiveData.TMALiveData mLiveData = new TMLiveData.TMALiveData(tmaLiveDataRoot);
mResult = null;
DataServiceQuery<TMLiveData.JobType> query = (DataServiceQuery<TMLiveData.JobType>)mLiveData.JobTypes.Where(c => c.IsActive == true);
mResult = "Trying to READ the data";
try
{
query.BeginExecute(OnQueryComplete, query);
}
catch (Exception ex)
{
mResult = "Error on beginExecute: " + ex.Message;
}
}
private static void OnQueryComplete(IAsyncResult result)
{
DataServiceQuery<TMLiveData.JobType> query = result as DataServiceQuery<TMLiveData.JobType>;
mResult = "Done!";
try
{
foreach (TMLiveData.JobType jobType in query.EndExecute(result))
{
mResult += jobType.JobType1 + ",";
}
}catch (Exception ex)
{
mResult = "Error looping for items: " + ex.Message;
}
}
}
}
有什么明显的我做错了吗?
您正在获得NullReferenceException
,因为您试图将IAsyncResult
转换为DataServiceQuery<TMLiveData.JobType>
。您需要转换IAsyncResult.AsyncState
:
private static void OnQueryComplete(IAsyncResult result)
{
DataServiceQuery<TMLiveData.JobType> query = (DataServiceQuery<TMLiveData.JobType>) result.AsyncState;
mResult = "Done!";
try
{
foreach (TMLiveData.JobType jobType in query.EndExecute(result))
{
mResult += jobType.JobType1 + ",";
}
}
catch (Exception ex)
{
mResult = "Error looping for items: " + ex.Message;
}
}
附带说明,我在这里使用了显式强制转换而不是as
操作符。如果您这样做了,您将得到InvalidCastException
而不是NullReferenceException
,这将使您更容易发现错误。