如何将匿名类型转换为IQueryable< Order_Detail&gt ;? [英] How to cast an anonymous type to IQueryable<Order_Detail>?

问题描述

我正在创建一个OData服务方法，在其中编写了以下代码:

I am creating an OData service method in which I have written the following code:

[WebGet]
public IQueryable<Order_Detail> getByYear(int year)
{
    var dc = new NorthwindBigEntities();
    var query = from p in dc.Order_Details
                where p.Order.OrderDate != null && p.Order.OrderDate.Value.Year == year
                select new
                {
                    TotalSales = p.UnitPrice * p.Quantity,
                    Product = p.Product.ProductName
                };
    return query;
}

但我遇到一个例外:

不能将类型'System.Linq.IQueryable AnonymousType#1'隐式转换为'System.Linq.IQueryable CustomMethod.Order_Detail'.

Cannot implicitly convert type 'System.Linq.IQueryable AnonymousType#1' to 'System.Linq.IQueryable CustomMethod.Order_Detail'.

我该怎么做?

推荐答案

代替select new进行select p.当前，您正在选择匿名对象，并且无法将其转换为System.Linq.IQueryable CustomMethod.Order_Detail

Instead of select new do select p. Currently you are selecting anonymous object and you can't convert it to System.Linq.IQueryable CustomMethod.Order_Detail

您无法投影到Order_Detail，因为它似乎是一类框架.由于您正在匿名对象中进行计算，因此您可以按select p返回IQueryable<Order_Detail>并稍后在LINQ上对对象进行计算，也可以为该对象创建一个新类和项目.如果最终创建一个新类，则应按照类名称修改签名.

You can't project to Order_Detail since it appears to be a class of framework. Since you are doing calculation in your anonymous object, You can either return IQueryable<Order_Detail> by select p and do the calculation later on LINQ to object, or you may create a new class and project to that. If you end up creating a new class then you should modify your signature as per class name.

public IQueryable<Order_Detail> getByYear(int year)
{
    var dc = new NorthwindBigEntities();
    var query = from p in dc.Order_Details
                where p.Order.OrderDate != null && p.Order.OrderDate.Value.Year == year
                select p;
    return query;
}

如果您创建一个新的类，例如:

If you create a new class like:

public class MyClass 
{
    public string Product {get;set;}
    public double TotalSales {get;set;}
}

那你就可以做

public IQueryable<MyClass> getByYear(int year)
{
    var dc = new NorthwindBigEntities();
    var query = from p in dc.Order_Details
                where p.Order.OrderDate != null && p.Order.OrderDate.Value.Year == year
                select new MyClass
                {
                    TotalSales = p.UnitPrice * p.Quantity,
                    Product = p.Product.ProductName
                };
    return query;
}

考虑返回数据的服务，如果您创建自己的类并仅返回所需的属性，则更好.您还应该看到此问题

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