Question

我有一个这样的课程：

public class studentpres
{
  public string stuname { set; get; }
  public  string stlastaname { set; get; }
  public string stunumber { set; get; }
  public string stumajor { set; get; }
  public string stufiled { set; get; }
  public string stuaverage { set; get; }
  public string stumobile { set; get; }
  public string stuemail { set; get; }
  public string stuprofSupervisor { set; get; }
  public string prifsuperEmail { set; get; }
}

我这样做一个查询：

public studentpres Get_Student_List(string stuNumber)
{
    studentpres temp =
        from i in dbconnect.tblUsers
        join d in dbconnect.tblNovitiates
            on i.tblStudent.studentNumber equals d.studentNumber
        select new PresentClass.studentpres()
        {
            prifsuperEmail = d.profSupervisorUSername,
            stlastaname = i.family,
            stuaverage = i.tblStudent.average,
            stuemail = i.email,
            stufiled = i.tblStudent.field,
            stumajor = i.tblStudent.major,
            stumobile = i.mobile,
            stuname = i.name,
            stunumber = i.tblStudent.studentNumber,
            stuprofSupervisor = Return_Name_By_userName(d.profSupervisorUSername)
        };
}

但我收到了这个错误：

无法隐式转换类型
'System.Linq.IQueryable<Novitiate.AdminPortal.PresentationClass.PresentClass.studentpres>' 至 'Novitiate.AdminPortal.PresentationClass.PresentClass.studentpres'。存在显式转换（您是否错过了演员？）

我还在查询结尾添加了 Tolist（），但它没有再次运行; 感谢

Answer 1

您的查询返回IQueryable<studentpres>，您应该只选择一条记录，使用：Single，First或其他方法：

单
的SingleOrDefault
第一
FirstOrDefault

Answer 2

您的查询返回IQueryable<studentpres>结果，但您尝试将其分配给studentpres类型的变量。如果您期望得到很多结果，那么将方法的返回类型（从方法名称我假设您需要这样做）更改为IQueryable<studentpres>或IEnumerable<studentpres>：

public IQueryable<studentpres> Get_Student_List(string stuNumber)
{
    return from i in dbconnect.tblUsers
           join d in dbconnect.tblNovitiates 
                    on i.tblStudent.studentNumber equals d.studentNumber
           select new PresentClass.studentpres()
           {
               prifsuperEmail = d.profSupervisorUSername,
               stlastaname = i.family,
                   // ...
           };
}

注意 - IQueryable<studentpress>将返回查询而不是查询执行的结果。如果要获取查询结果，则应执行它（即将查询结果保存到列表中）：

public IList<studentpres> Get_Student_List(string stuNumber)
{
    var query = from i in dbconnect.tblUsers
                join d in dbconnect.tblNovitiates 
                    on i.tblStudent.studentNumber equals d.studentNumber
                select new PresentClass.studentpres()
                {
                   prifsuperEmail = d.profSupervisorUSername,
                   stlastaname = i.family,
                   stuaverage = i.tblStudent.average,
                   stuemail = i.email,
                   stufiled = i.tblStudent.field,
                   stumajor = i.tblStudent.major,
                   stumobile = i.mobile,
                   stuname = i.name,
                   stunumber = i.tblStudent.studentNumber,
                   stuprofSupervisor = 
                      Return_Name_By_userName(d.profSupervisorUSername)
                };

      return query.ToList(); // executes query
}

如果您想获得单个结果，请在查询时拨打FistOrDefault()或SingleOrDefault()：

public studentpres Get_Student_List(string stuNumber)
{
    var query = from i in dbconnect.tblUsers
                join d in dbconnect.tblNovitiates 
                    on i.tblStudent.studentNumber equals d.studentNumber
                select new PresentClass.studentpres()
                {
                   prifsuperEmail = d.profSupervisorUSername,
                   stlastaname = i.family,
                   // ...
                };

    return query.FirstOrDefault();
}

将select查询映射到对象会在linq中出错

2 个答案: