LINQ动态组合查询

 最近在项目中用到了LINQ，在界面上有许多组合查询条件，我是一个比较懒的人，呵呵，不想在数据查询层写许多方法。我自己尝试写了个Expression的LINQ动态查询，目的是到达了，但是我在代码的初始表达式为null，每次组合AND前都要判断是否为空，为空则返回右边的表达式。今天在网上Google了一下，看到肖坤：Linq动态查询与模糊查询（带源码示例）中讲到的《dynamic linq queries / dynamic where clause (part 2) 》，中老外写的PredicateExtensions类。便用这个类修改了下自己的方案。不在是全表达式树动态生成了。今天特地写了一个基于Northwind的Demo拿来和大家分享：

效果贴图：

   界面是用Telerik控件做的（控件对于我们后台开发人员来说就是少调样式）。

其中其实很简单，主要方法就两个，一个事表达式树组合，和数据绑定。数据绑定，有一个

Expression<Func<Orders, bool>> 的where查询条件。

Controller类Code

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();

    }

}




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我的排序辅助类：

代码

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
    }

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老外的PredicateExtensions类很简单，只是真的思路很优秀，看到代码我们都会恍然大悟，

但是估计我这鼠辈很难想到。

代码

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);

    }

}




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上面贴图条件输出框中输出的sql我也截了一个图来供参考：
 

sql看不全：故在这里也贴一个：

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

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在这里说老外的思想优秀，该讲解一下，但是限于篇幅，还有时间凌晨1点了，故改到下回讲解。

附带：

本随笔代码下载

：代码
 

代码：

 

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);
    }
}

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