PredicateBuilder類（linq多條件組合查詢）

PredicateBuilder類如下：

 public static class PredicateBuilder
    {

        /// <summary>
        /// 機關函數應用True時：單個AND有效，多個AND有效；單個OR無效，多個OR無效；混應時寫在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>
        /// 機關函數應用False時：單個AND無效，多個AND無效；單個OR有效，多個OR有效；混應時寫在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);
        } 
    }

　　多條件查詢的代碼：

/// <summary>
    /// 多條件查詢
    /// </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();
        }
    }

  上面代碼中，txtName是姓名文本框，txtAge是年齡文本框，因為要進行and條件查詢所以一開始使用PredicateBuilder.True<StuInfo>()來創建初始為true的where條件，

如果進行or多條件查詢，就應該使用PredicateBuilder.False<StuInfo>()來創建初始為false的where條件