如何投匿名类型的IQueryable< Order_Detail>在C＃中？ [英] how to cast an anonymous type to IQueryable<Order_Detail> in C#?

问题描述

我创建了一个 OData服务方法，其中我已编写以下代码

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

但我遇到异常

无法将类型System.Linq.IQueryable AnonymousType＃1隐式转换为System.Linq.IQueryable CustomMethod.Order_Detail。

如何完成

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解决方案c> select p 。目前您选择的是匿名对象，不能将其转换为 System.Linq.IQueryable CustomMethod.Order_Detail

您不能投影到 Order_Detail ，因为它似乎是一类框架。由于您在匿名对象中进行计算，您可以通过 select p 返回 IQueryable< Order_Detail> 稍后计算LINQ到对象，或者你可以创建一个新的类和项目。如果你最终创建一个新类，那么你应该修改签名为每个类名。

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

如果您创建一个新类：

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

然后您可以执行

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

考虑它返回数据的服务，自己的类，并仅返回所需的属性。您还应该看到此问题

I am creating an OData service method in which I have written 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;
        }

but I am getting an exception

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

how can I accomplish this ??

解决方案

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

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;
}

If you create a new class like:

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

Then you can do

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;
}

Considering its a service returning data, its better if you create your own class and return only the required properties. You should also see this question

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