linq to sql的多條件動态查詢(下)

借助老外寫的一個擴充表達式的類，可以把上篇中的代碼寫得更優雅

這是PredicateBuilder的源檔案

 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)

              (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));

        }

        //翻譯後的sql語句:

        //Select [t0].[ID], [t0].[Name], [t0].[Description]

        //FROM [dbo].[Product] AS [t0]

        //Where ([t0].[Description] LIKE '%手機%') AND ([t0].[Description] LIKE '%6111%')

        return query.ToList();   

    List<Product> GetProductsByOR(params string[] keywords)

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

            predicate = predicate.Or(p => p.Description.Contains(temp));

        var query = db.Products.Where(predicate);

        //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 = "長虹";

        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();