LINQ动态条件查询问题？
这两天在研究LINQ动态查询，但是自己把网上拿来东西在根据自己需要在变通一下就有问题了，请帮忙解决下，我已在本机上调试很多次都有问题的~
IQueryable<Customer> custs = db.Customers; 改成IQueryable<Customer> custs = db.Customers.Where(c=>c.City=="London");//先筛选再动态查询ParameterExpression param = Expression.Parameter(typeof(Customer), "c");Expression selector = Expression.Property(param, typeof(Customer).GetProperty("ContactName"));Expression pred = Expression.Lambda(selector, param);Expression expr = Expression.Call(typeof(Queryable), "Select", new Type[] { typeof(Customer), typeof(string) }, Expression.Constant(custs), pred);IQueryable<string> query = db.Customers.AsQueryable() .Provider.CreateQuery<string>(expr); //经过上面的筛选调试结果为空？请问能否这样？System.Data.Common.DbCommand cmd = db.GetCommand(query);Console.WriteLine(cmd.CommandText);IQueryable<Customer> custs = db.Customers;ParameterExpression param = Expression.Parameter(typeof(Customer), "c");Expression left = Expression.Property(param, typeof(Customer).GetProperty("City"));Expression right = Expression.Constant("London");Expression filter = Expression.Equal(left, right);Expression pred = Expression.Lambda(filter, param);MethodCallExpression whereCallExpression = Expression.Call( typeof(Queryable), "Where", new Type[] { typeof(Customer) }, Expression.Constant(custs), pred);//OrderBy(ContactName => ContactName)MethodCallExpression orderByCallExpression = Expression.Call( typeof(Queryable), "OrderBy", new Type[] { typeof(Customer), typeof(string) }, whereCallExpression, Expression.Lambda(Expression.Property (param, "ContactName"), param));//生成动态查询IQueryable<Customer> query = db.Customers.AsQueryable() .Provider.CreateQuery<Customer>(orderByCallExpression);