How to cast an anonymous type to IQueryable<Order_Detail>?

问题

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;
}

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?

回答1:

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

来源：https://stackoverflow.com/questions/16734320/how-to-cast-an-anonymous-type-to-iqueryableorder-detail