LINQ动态组合查询

using  System;
using  System.Web.UI;
using  System.Linq.Expressions;
  public   partial   class  _Default : System.Web.UI.Page
{
     private  OrderController controller  =   new  OrderController();
     protected   void  Page_Load( object  sender, EventArgs e)
    {
         if  ( ! IsPostBack)
        {
            Bindgrid( null );
        }
    } 
     public   void  Bindgrid(Expression < Func < Orders,  bool >>  whereqQuery)
    {
        grid.DataSource  =  controller.GetOrders( 1 ,  15 , whereqQuery,
  new  QueryableOrderEntry < Orders, 
string > (t  =>  t.CustomerID, OrderDirection.DESC)); //  分页数目应该用分页控件，
这里只是简单测试，所以偷个懒
        grid.DataBind();
    }
      protected   void  btnSearch_Click( object  sender, ImageClickEventArgs e)
    {
        Expression < Func < Orders,  bool >>  expression  =  PredicateExtensions.True < Orders > ();
         if  ( ! string .IsNullOrEmpty(txtCustomerId.Text.Trim()))
        {
             string  str  =  txtCustomerId.Text.Trim();
            expression  =  expression.And(o  =>  o.CustomerID.Contains(str));
        }
         if  ( ! string .IsNullOrEmpty(txtEmplyeeId.Text.Trim()))
        {
             string  str  =  txtEmplyeeId.Text.Trim();
            expression  =  expression.And(o  =>  o.EmployeeID.HasValue  &&  
                o.EmployeeID.Value.Equals( int .Parse(str)));
        }
         if  (txtOrderDateStart.SelectedDate.HasValue)
        {DateTime dt = txtOrderDateStart.SelectedDate.Value;
            expression  =  expression.And(o  =>  o.OrderDate.HasValue
  &&  o.OrderDate.Value  >=  dt);
        }
 
         if  (txtOrderDateEnd.SelectedDate.HasValue)
        {
            DateTime dt  =  txtOrderDateEnd.SelectedDate.Value;
            expression  =  expression.And(o  =>  o.OrderDate.HasValue
  &&  o.OrderDate.Value  <= dt);
        }
        Bindgrid(expression); 
    }
}

using  System;
using  System.Collections.Generic;
using  System.Linq;
using  System.Web;
using  System.Linq.Expressions;
using  System.Diagnostics; 
public   class  OrderController
{
     protected  NorthwindDataContext DataContext
    {
         get
        {
            NorthwindDataContext context  =  
HttpContext.Current.Items[ " NorthwindDataContext " ]
  as  NorthwindDataContext;
             if  (context  ==   null )
            {
                context  =   new  NorthwindDataContext();
                HttpContext.Current.Items[ " NorthwindDataContext " ]  =  context;
            }
             return  context;
        }
    }
      public  List < Orders >  GetOrders < TKey > ( int  currentPage,
  int  pagesize, Expression < Func < Orders,  bool >>  whereQuery, 
QueryableOrderEntry < Orders, TKey >  orderQuery)
// 重点在这里的参数 
    {
        IQueryable < Orders >  query  =  DataContext.Orders;
         if  (whereQuery  !=   null )
        {
            query  =  query.Where(whereQuery);
        }
         if  (orderQuery  !=   null )
        {
             if  (orderQuery.OrderDirection  ==  OrderDirection.ASC)
            {
               query =  query.OrderBy(orderQuery.Expression);
            }
             else
            {
                query  =  query.OrderByDescending(orderQuery.Expression);
            }
        }
        Debug.WriteLine(DataContext.GetCommand(query.Skip((currentPage  -   1 )  *  pagesize)
            .Take(pagesize)).CommandText);
         return  query.Skip((currentPage  -   1 )  *  pagesize).Take(pagesize).ToList();
    }
 
     public   void  Save()
    {
        DataContext.SubmitChanges();
    }
}

using  System;
using  System.Linq.Expressions;
     public   class  QueryableOrderEntry < TSource, TKey >
    {
 
         public  QueryableOrderEntry(Expression < Func < TSource, TKey >>  expression)
        {
             this .Expression  =  expression;
            OrderDirection  =  OrderDirection.ASC;
        }
 
         public  QueryableOrderEntry(Expression < Func < TSource, TKey >>  expression, 
                  OrderDirection orderDirection)
        {
             this .Expression  =  expression;
            OrderDirection  =  orderDirection;
        }
         public  Expression < Func < TSource, TKey >>  Expression
        {
             get ;
             set ;
        }
 
         public  OrderDirection OrderDirection
        {
             get ;
             set ;
        }
    }
     public   enum  OrderDirection
    {
        ASC, DESC
    }

public   static   class  PredicateExtensions
{
     public   static  Expression < Func < T,  bool >>  True < T > () {  return  f  =>   true ; }
 
     public   static  Expression < Func < T,  bool >>  False < T > () {  return  f  =>   false ; }
 
     public   static  Expression < Func < T,  bool >>  Or < T > ( this  Expression < Func < T,  bool >>  expression1,
            Expression < Func < T,  bool >>  expression2)
    {
        var invokedExpression  =  Expression.Invoke(expression2, 
expression1.Parameters.Cast < Expression > ());
 
         return  Expression.Lambda < Func < T,  bool >> (Expression.Or(
expression1.Body, invokedExpression), 
           expression1.Parameters);
    }
 
     public   static  Expression < Func < T,  bool >>  And < T > ( this  Expression < Func < T,  bool >>  expression1, 
            Expression < Func < T,  bool >>  expression2)
    {
        var invokedExpression  =  Expression.Invoke(expression2, 
expression1.Parameters.Cast < Expression > ());
 
         return  Expression.Lambda < Func < T,  bool >> (Expression.And(expression1.Body, 
          invokedExpression), expression1.Parameters);
    }
}

SELECT   TOP  ( 15 )  [ t0 ] . [ OrderID ] ,  [ t0 ] . [ CustomerID ] ,  [ t0 ] . [ EmployeeID ] ,  [ t0 ] . [ OrderDate ] ,
  [ t0 ] . [ RequiredDate ] ,  [ t0 ] . [ ShippedDate ] ,  [ t0 ] . [ ShipVia ] ,  [ t0 ] . [ Freight ] ,  [ t0 ] . [ ShipName ] ,
  [ t0 ] . [ ShipAddress ] ,  [ t0 ] . [ ShipCity ] ,  [ t0 ] . [ ShipRegion ] ,  [ t0 ] . [ ShipPostalCode ] , 
[ t0 ] . [ ShipCountry ] FROM   [ dbo ] . [ Orders ]   AS   [ t0 ]
WHERE  ( [ t0 ] . [ CustomerID ]   LIKE   @p0 )  AND  ( [ t0 ] . [ EmployeeID ]   IS   NOT   NULL )  AND  
(( [ t0 ] . [ EmployeeID ] )  =   @p1 )  AND  ( [ t0 ] . [ OrderDate ]   IS   NOT   NULL )  AND  
(( [ t0 ] . [ OrderDate ] )  >=   @p2 )  AND  ( [ t0 ] . [ OrderDate ]   IS   NOT   NULL )  AND  
(( [ t0 ] . [ OrderDate ] )  <=   @p3 ) ORDER   BY   [ t0 ] . [ CustomerID ]   DESC