Question

我只需要返回第一条记录。虽然我不得不把z.FirstOrDefault（）;还有不止一个人出现了

           public DataTable GetDependents(string EmployeeID)
    {
        HealthCareSystem.DataClassesDataContext db = new HealthCareSystem.DataClassesDataContext();

        var z = (from s in db.SelectingDependentsGroupBies
                 where s.EmployeeID.Equals(EmployeeID)
                 join d in db.Dependents on s.DependentID equals d.DependentID
                 orderby s.DependentID descending

                 //Selecting wanted tables dependents fields by datatable
                 select new DependentsX { DependentID = Convert.ToInt32(s.DependentID), EmployeeID = s.EmployeeID, Name = s.Name, Surname = s.Surname, IDCardNo = s.IDCardNo, ContactNo = s.ContactNo, BirthDate = s.BirthDate, StartSchemeDate = s.StartDate, EndSchemeDate = s.EndDate, RelationType = s.Type, Payment = Convert.ToDouble(d.Payment), });

        var firstRecord = z.FirstOrDefault();

Answer 1

在现有代码之后添加此行：

var firstRecord = x.First();

如果结果可能为零且您不想要例外，那么您可以改为使用FirstOrDefault。

var firstRecord = x.FirstOrDefault();

Answer 2

另一种常见方法是将查询括在括号中，然后在括号外包含扩展方法：

var x = (from d in db.DependentsRelationsViews 
        where d.IDCardNo.Equals(DepID) 
        orderby d.DependentID descending 
        select new DependentsX  
        {  
            DependentID = Convert.ToInt32(d.DependentID),  
            EmployeeID = d.EmployeeID,  
            Name = d.Name,  
            Surname = d.Surname,  
            IDCardNo = d.IDCardNo,  
            ContactNo = d.ContactNo,  
            BirthDate = d.BirthDate,  
            StartSchemeDate = Convert.ToDateTime(d.StartSchemeDate),  
            EndSchemeDate = d.EndSchemeDate,  
            Payment = d.Payment,  
            RelationType = Convert.ToString(d.Type) 
        }).FirstOrDefault();

如果你没有将它括在括号中并尝试调用FirstOrDefault()，那么你只是在最后一个Select子句中将该扩展应用于匿名类型。通过将整个查询括在括号中然后调用FirstOrDefault()扩展，您将向编译器指示您希望将其应用于整个查询的返回值，即IEnumerable<>或IQueryable<>集合。

需要返回第一条记录linq

2 个答案: