如何在linq中选择多个条件？

Question

我使用 MVC 4，我认为有 5 个下拉列表。当下拉列表选择文本并单击按钮搜索时，我想从选择下拉列表的 4 个表中进行选择。如果没有选中下拉列表，则未选中。

(from met in db.tblMet
                         from useMet in db.tblUseMet.Where(m => m.UseMetID == met.UseMetID_FK).DefaultIfEmpty()
                         from typeMet in db.tblTypeMet.Where(m => m.TypeMetID == met.TypeMetID_FK).DefaultIfEmpty()
                         from mod in db.tblMod.Where(m => m.ModID == met.ModID_FK).DefaultIfEmpty()
                         from affair in db.tblAffairs.Where(m => m.AffairID == met.AffairID_FK).DefaultIfEmpty()
                         from desert in db.tblDeserts.Where(m => m.DesertID == met.DesertID_FK).DefaultIfEmpty()
                         from city in db.tblCities.Where(m => m.CityID == met.CityID_FK).DefaultIfEmpty()
                         from user in db.tblUsers.Where(m => m.UserID == met.UserIDCreate_FK).DefaultIfEmpty()
                         from userCh in db.tblUsers.Where(m => m.UserID == met.UserIDChange_FK).DefaultIfEmpty()
                         from setting in db.tblSettings.Where(m => m.SettingID == met.SettingID_FK).DefaultIfEmpty()
                         from sim in db.tblSims.Where(m => m.SimID == mod.SimID_FK).DefaultIfEmpty()
                         from typemod in db.tblTypeMod.Where(m => m.TypeModID == sim.TypeModID_FK_Kind).DefaultIfEmpty()
                         from gro in db.tblGroupMet.Where(m => m.GroupMetID == met.GroupMetID_FK).DefaultIfEmpty()
                         from group1 in db.tblMetRelateGroups.Where(x => x.MetID_FK == met.MetID).DefaultIfEmpty()
                         from status in db.tblStatus1.Where(m => m.StatusID == met.StatusID_FK).DefaultIfEmpty()

                         where ((city.CityID == City1||city.CityID !=null)
                               && (typeMet.TypeMetID == Type1 || typeMet.TypeMetID != null)
                               && (useMet.UseMetID == Usemeter1|| useMet.UseMetID != null)
                               && (group1.GroupMetID_FK ==Group1 || group1.GroupMetID_FK != null)
                               && (affair.AffairID ==Affair1 || affair.AffairID != null)
                             //|| desert.DesertID==Desert1
                               )

我想选中已选中的下拉列表，如果不选择任何，则未签入查询。在上面的代码中选择零记录。

Answer 1

您只需为此目的使用流利的语法。这是一个例子：

var lQuery = from met in db.tblMet;

if (City1 != null)
  lQuery = lQuery.Where(r => r.CityID == City1);

...

通过这种方式，您可以为查询动态添加条件。

如果您想使用逻辑 OR 添加条件，您需要一个谓词构建器，请参阅https://stackoverflow.com/a/1775057/3936440。