Linq中动态生成查询

查询数据的过程中,如果需要根据某些条件的存在与否来查询,那么动态生成条件是必不可少的:

            //构造一个查询及表达式树
            IQueryable qy = cont.Category;
            ParameterExpression param = Expression.Parameter(typeof(Category), "c");

            //条件1
            Expression right = Expression.Constant(10);
            Expression left = Expression.Property(param, typeof(Category).GetProperty("ID"));
            Expression p1 = Expression.Equal(left, right);

            //组合条件2
            if (checkBox1.Checked)
            {
                Expression right1 = Expression.Constant(11);
                Expression filter1 = Expression.Equal(left, right1);
                p1 = Expression.Or(p1, filter1);
            }

            Expression pred = Expression.Lambda(p1, param);

            //生成查询
            Expression expr = Expression.Call(typeof(Queryable), "Where", new Type[] { typeof(Category) }, Expression.Constant(qy), pred);
            IQueryable<Category> query = cont.Category.AsQueryable().Provider.CreateQuery<Category>(expr);
            this.dataGridView1.DataSource = query;