SPBuiltInFieldId class

public sealed class SPBuiltInFieldId

using System;
using System.Data;
using Microsoft.SharePoint;

namespace Test
{
   class ConsoleApp
   {
      static void Main(string[] args)
      {
         using (SPSite site = new SPSite("http://localhost"))
         {
            using (SPWeb web = site.OpenWeb())
            {
               // Get IDs for all fields used in the query.
               string assignedToId = SPBuiltInFieldId.AssignedTo.ToString("B");
               string taskDueDateId = SPBuiltInFieldId.TaskDueDate.ToString("B");
               string titleId = SPBuiltInFieldId.Title.ToString("B");
               string taskStatusId = SPBuiltInFieldId.TaskStatus.ToString("B");
               string percentCompleteId = SPBuiltInFieldId.PercentComplete.ToString("B");

               // Define the data selection.
               string where = "<Where><Eq>";
               where += "<FieldRef ID='" + assignedToId + "' />";
               where += "<Value Type='Integer'><UserID/></Value>";
               where += "</Eq></Where>";

               // Define the sort order.
               string orderBy = "<OrderBy>";
               orderBy += "<FieldRef ID='" + taskDueDateId + "' />";
               orderBy += "</OrderBy>";

               SPSiteDataQuery query = new SPSiteDataQuery();

               // Set the query string.
               query.Query = where + orderBy;

               // Query task lists.
               query.Lists = "<Lists ServerTemplate='107'/>";

               // Specify the view fields.
               query.ViewFields = "<FieldRef ID='" + titleId + "' />";
               query.ViewFields += "<FieldRef ID='" + taskDueDateId + "' Nullable='TRUE' />";
               query.ViewFields += "<FieldRef ID='" + taskStatusId + "' Nullable='TRUE' />";
               query.ViewFields += "<FieldRef ID='" + percentCompleteId + "' Nullable='TRUE' />";

               // Query all Web sites in this site collection.
               query.Webs = "<Webs Scope='SiteCollection'>";

               // Run the query.
               DataTable results = web.GetSiteData(query);

               // Print the results.
               Console.WriteLine("{0, -10} {1, -30} {2, -30} {3}", "Date Due", "Task", "Status", "% Complete");
               foreach (DataRow row in results.Rows)
               {
                  // Extract column values from the data table.
                  string dueDate = (string)row[taskDueDateId];
                  string task = (string)row[titleId];
                  string status = (string)row[taskStatusId];
                  string percentComplete = (string)row[percentCompleteId];

                  // Convert the due date to a short date string.
                  DateTime dt;
                  bool hasDate = DateTime.TryParse(dueDate, out dt);
                  if (hasDate)
                     dueDate = dt.ToShortDateString();
                  else
                     dueDate = String.Empty;

                  // Convert the PercentComplete field value to a percentage.
                  decimal pct;
                  bool hasValue = decimal.TryParse(percentComplete, out pct);
                  if (hasValue)
                     percentComplete = pct.ToString("P0");
                  else
                     percentComplete = "0 %";

                  // Print a line.
                  Console.WriteLine("{0, -10} {1, -30} {2, -30} {3, 10}", dueDate, task, status, percentComplete);
               }
            }
         }
         Console.ReadLine();
      }
   }
}