public static class PredicateBuilder    {        /// <summary>        /// 机关函数应用True时：单个AND有效，多个AND有效；单个OR无效，多个OR无效；混应时写在AND后的OR有效          /// </summary>        /// <typeparam name="T"></typeparam>        /// <returns></returns>        public static Expression<Func<T, bool>> True<T>() { return f => true; }        /// <summary>        /// 机关函数应用False时：单个AND无效，多个AND无效；单个OR有效，多个OR有效；混应时写在OR后面的AND有效          /// </summary>        /// <typeparam name="T"></typeparam>        /// <returns></returns>        public static Expression<Func<T, bool>> False<T>() { return f => false; }        public static Expression<Func<T, bool>> Or<T>(this Expression<Func<T, bool>> expr1,                                                            Expression<Func<T, bool>> expr2)        {            var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());            return Expression.Lambda<Func<T, bool>>                  (Expression.Or(expr1.Body, invokedExpr), expr1.Parameters);        }        public static Expression<Func<T, bool>> And<T>(this Expression<Func<T, bool>> expr1,                                                             Expression<Func<T, bool>> expr2)        {            var invokedExpr = Expression.Invoke(expr2, expr1.Parameters.Cast<Expression>());            return Expression.Lambda<Func<T, bool>>                  (Expression.And(expr1.Body, invokedExpr), expr1.Parameters);        }     }

/// <summary>    /// 多条件查询    /// </summary>    /// <param name="sender"></param>    /// <param name="e"></param>    protected void btnSearch_Click(object sender, EventArgs e)    {        using(LinqDBDataContext db = new LinqDBDataContext())        {            var list = db.StuInfo;            var where = PredicateBuilder.True<StuInfo>();            if(this.txtName.Text.Trim().Length!=0)            {                where = where.And(p => p.StuName.Contains(this.txtName.Text.Trim()));            }            if(this.txtAge.Text.Trim().Length!=0)            {                where = where.And(p => p.StuAge == Convert.ToInt32(this.txtAge.Text.Trim()));            }            var result = list.Where(where).ToList();            this.repStuInfo.DataSource = result;            this.repStuInfo.DataBind();        }    }