如何在linq中選擇多個條件？

Question

我使用MVC 4，並且在我的視圖中有5個下拉列表。 當下拉列表選擇文本並單擊按鈕搜索時，我想從選擇下拉列表的4個表中進行選擇。 如果沒有選擇下拉列表，則不簽入條件。

(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
                               )

我想選擇下拉列表，如果不選擇，則不在查詢中。 在上面的代碼中選擇零記錄。

Answer 1

您只需為此使用流利的語法。 這是一個例子：

var lQuery = from met in db.tblMet;

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

...

這樣，您可以為查詢動態添加條件。

如果要使用邏輯或添加條件，則需要謂詞構建器，請參閱https://stackoverflow.com/a/1775057/3936440 。