TFS API C#：获取工作项的问题

using Microsoft.TeamFoundation.Client;

// TFS服务器的URL
string tfsUrl = "http://tfs-server-url";

// 用户凭据
string username = "username";
string password = "password";

// 连接到TFS服务器
TfsTeamProjectCollection tfs = new TfsTeamProjectCollection(new Uri(tfsUrl), new NetworkCredential(username, password));

using Microsoft.TeamFoundation.WorkItemTracking.Client;

// 获取工作项
WorkItemStore workItemStore = tfs.GetService<WorkItemStore>();

// 根据工作项的ID获取工作项
int workItemId = 123;
WorkItem workItem = workItemStore.GetWorkItem(workItemId);

// 根据查询条件获取工作项
string query = "SELECT [System.Id], [System.Title] FROM WorkItems WHERE [System.WorkItemType] = 'Bug'";
WorkItemCollection workItems = workItemStore.Query(query);

// 获取工作项的问题字段的值
string issue = workItem.Fields["System.Description"].Value.ToString();