linq to sql 中,如何解决多条件查询问题,答案,用表达式树! (下)

复制代码 代码如下:

using System.Linq.Expressions;

public static class PredicateExtensionses
{
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>> exp_flow, Expression<Func<T, bool>> expression2)
{

var invokedExpression = System.Linq.Expressions.Expression.Invoke(expression2, exp_flow.Parameters.Cast<System.Linq.Expressions.Expression>());

return System.Linq.Expressions.Expression.Lambda<Func<T, bool>>(System.Linq.Expressions.Expression.Or(exp_flow.Body, invokedExpression), exp_flow.Parameters);

}
public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> exp_flow, Expression<Func<T, bool>> expression2)
{

var invokedExpression = System.Linq.Expressions.Expression.Invoke(expression2, exp_flow.Parameters.Cast<System.Linq.Expressions.Expression>());

return System.Linq.Expressions.Expression.Lambda<Func<T, bool>>(System.Linq.Expressions.Expression.And(exp_flow.Body, invokedExpression), exp_flow.Parameters);

}

}

复制代码 代码如下:

private void ListDataBind(int pageIndex)
{
int rowCount = 0;
int pageCount = 0;
int pageSize = 30;
Expression<Func<FeedBack, bool>> expr = PredicateExtensionses.True<FeedBack>();
GetCondition(ref expr);
var hs = from h in hm.AllFeedBacks.Where(expr) select h;//延迟加载,数据库没有任何操作
if (pageIndex == 1)//如果是第一次取数据,需要获取符合条件的总记录条数
{
rowCount = hs.Count();//数据库进行一次Count操作
}
else//之后的记录条数,从分页控件持久态的属性中获取,省去一次Count查询
{
rowCount = PageNavigator1.RecordCount;
}
pageCount = rowCount > pageSize ? Convert.ToInt32((rowCount - 1) / pageSize) + 1 : 1;//通用分页算法
if (pageIndex > pageCount)
{
pageIndex = pageCount;
}
var pageData = hs.Skip(pageSize * (pageIndex - 1)).Take(pageSize);//这里也是延迟加载,数据库此时不操作
FeedBackManageList.DataSource = pageData;//这里才正式加载数据,仅仅向数据库发出请求30条记录SQL
FeedBackManageList.DataBind();
PageNavigator1.RecordCount = rowCount;// 给分页控件一些数据
PageNavigator1.PageCount = pageCount;//给分页控件一些数据
PageNavigator1.PageIndex = pageIndex;//给分页控件一些数据
}

复制代码 代码如下:

private void GetCondition(ref Expression<Func<FeedBack, bool>> expr) {
int isLock = Int32.Parse(ddlIsLock.SelectedValue);
if (isLock > -1)
{
expr = expr.And(c => (c.IsLock == isLock));//一次组合
}
string keyword = tbxKeyword.Text.FilterInjectStr();
if (!keyword.IsNullOrEmpty())
{
expr = expr.And(c => (c.HotelName.IndexOf(keyword) > -1)); //二次组合
}
}