Linq动态条件查询

1、添加类PredicateExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Linq.Expressions;

namespace Www24100Net.PlayingWithLinq
{
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);

}

}

}

2、Linq动态查询
private void bind(int pageIndex)
{
if(pageIndex < 1){pageIndex = 1;}

var predicate = PredicateExtensions.True<CityName>();

string _fprovince = "福建";
if (!string.IsNullOrEmpty(_fprovince))
{
predicate = predicate.And(p => p.FProvince.Contains(_fprovince));
}

string _fcity = "厦门";
if (!string.IsNullOrEmpty(_fcity))
{
predicate = predicate.And(p => p.FCity.Contains(_fcity));
}

string _ftype = "联通";
if (!string.IsNullOrEmpty(_ftype))
{
predicate = predicate.And(p => p.FType.Contains(_ftype));
}

DataClassesDataContext DBCity = new DataClassesDataContext();

var query = DBCity.CityName.Where(predicate);

AspNetPager1.RecordCount = query.Count();
this.Label1.Text = AspNetPager1.RecordCount.ToString();

intStartRow = intPageSize * (pageIndex-1) ;
intEndRow = pageIndex * intPageSize;

GridView1.DataSource = query.OrderBy(p => p.FSimseg).Skip(intStartRow).Take(intPageSize);
GridView1.DataBind();
}