documentService.CreateWorkItem（workItem，a）显示“未将对象引用设置为对象的实例”。

Question

我正在尝试修改工作项字段并以务实的方式启动它。 下面是代码

ICredentials credentials = new NetworkCredential("user", "paswd", "domain");

        TfsTeamProjectCollection tfs = new TfsTeamProjectCollection(new Uri("path "), credentials);

        WorkItemStore workItemStore = (WorkItemStore)tfs.GetService<WorkItemStore>();


        WorkItemTypeCollection workItemTypes = workItemStore.Projects["project"].WorkItemTypes;

        WorkItemType workItemType = workItemTypes["Bug"];

        var workItem = new WorkItem(workItemType)
        {
            Title = "Test",
        };


        workItem.Fields["Microsoft.VSTS.Common.Priority"].Value = "2";
        workItem.Description = "New Description";

        documentService = (DocumentService)_applicationObject.DTE.GetObject("Microsoft.VisualStudio.TeamFoundation.WorkItemTracking.DocumentService");

        var a = new object();

        IWorkItemDocument workItemDocument = documentService.CreateWorkItem(workItem, a);

       (Throws exception : "Object reference not set to an instance of an object." in above line)


        documentService.ShowWorkItem(workItemDocument);

请建议如何处理异常

谢谢和问候， 钱德拉