多条件动态LINQ 组合查询

本文章转载：http://www.cnblogs.com/wangiqngpei557/archive/2013/02/05/2893096.html

参考：http://dotnet.9sssd.com/entfwk/art/960

 http://www.cnblogs.com/killuakun/archive/2008/08/03/1259389.html

http://www.cnblogs.com/snowdream/archive/2008/07/18/1246308.html

 

以往我们都是通过判断的方式来拼接查询的SQL字符串，但是现在我们面对是强类型的LINQ查询，是否可以很方便的进行类似查询。

 

eg:

string _UserID = string .Empty;
           _UserID = "E351D301-F64B-412C-B9EF-573F41235AF2" ;

           string _UserName = string .Empty;
           _UserName = "admin" ;

           string _employyName = string .Empty;
           _employyName = "测试1" ;

           using ( var xj = new XJGasBottles_testDataContext())
           {
               //Linq写法
               var usersLinq = from us in xj.Users
                               where ( string .IsNullOrEmpty(_UserID) || us.UserID.ToString() == _UserID)
                                      && ( string .IsNullOrEmpty(_UserName) || us.UserName == _UserName)
                                      || (us.EmpName == _employyName)
                               //where string.IsNullOrEmpty(_UserID) || us.UserID.ToString()==_UserID
                               //where string.IsNullOrEmpty(_UserName) || us.UserName==_UserName
                               select us;
               foreach ( var item in usersLinq)
               {
                   Console.WriteLine( "Linq:" );
                   Console.WriteLine(item.UserID + "_" + item.UserName);

               }

               //Lamda写法
               var usersLamda = xj.Users.Where(s => ( string .IsNullOrEmpty(_UserID) || s.UserID.ToString() == _UserID) &&
                                               ( string .IsNullOrEmpty(_UserName) || s.UserName == _UserName) ||
                                               (s.EmpName==_employyName)
                                         )
                                   .Select(s => s);

               foreach ( var item in usersLamda)
               {
                   Console.WriteLine( "Lamda:" );
                   Console.WriteLine(item.UserID + "_" + item.UserName);

               }

           }