linq to sql的多条件动态查询(下)

借助老外写的一个扩展表达式的类，可以把上篇中的代码写得更优雅

这是PredicateBuilder的源文件

 public static class PredicateBuilder
    ...{
      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>> 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);
      }
    }

下面是使用示例　：

List<Product> GetProductsByAND(params string[] keywords) 
    ...{
        DBDataContext db = new DBDataContext(Database.ConnectionString);
        IQueryable<Product> query = db.Products;
        foreach (string keyword in keywords)
        ...{
            string temp = keyword;
            query = query.Where(p => p.Description.Contains(temp));
        }
        //翻译后的sql语句:
        //Select [t0].[ID], [t0].[Name], [t0].[Description]
        //FROM [dbo].[Product] AS [t0]
        //Where ([t0].[Description] LIKE '%手机%') AND ([t0].[Description] LIKE '%6111%')
        return query.ToList();   
    }


    List<Product> GetProductsByOR(params string[] keywords)
    ...{
        DBDataContext db = new DBDataContext(Database.ConnectionString);
        var predicate = PredicateBuilder.False<Product>();
        foreach (string keyword in keywords)
        ...{
            string temp = keyword;
            predicate = predicate.Or(p => p.Description.Contains(temp));
        }
        var query = db.Products.Where(predicate);
        //翻译后的sql语句:
        //Select [t0].[ID], [t0].[Name], [t0].[Description]
        //FROM [dbo].[Product] AS [t0]
        //Where ([t0].[Description] LIKE '%6111%') OR ([t0].[Description] LIKE '%2350%')
        return query.ToList();
    }

    void ShowData() 
    ...{
        //var _products = GetProductsByOR("6111", "2350");
        //Repeater1.DataSource = _products;
        //Repeater1.DataBind();

        var predicate = PredicateBuilder.True<Product>();

        string _name = "6111";
        if (!string.IsNullOrEmpty(_name)) 
        ...{
            predicate = predicate.And(p => p.Name.Contains(_name));
        }

        string _description = "长虹";
        if (!string.IsNullOrEmpty(_description)) 
        ...{
            predicate = predicate.And(p => p.Description.Contains(_description));
        }

        using (DBDataContext db = new DBDataContext(Database.ConnectionString))
        ...{
            var _Products = db.Products.Where(predicate);
            Repeater1.DataSource = _Products;
            Repeater1.DataBind();
        }

    }