不能将类型 [concrete] 隐式转换为 [interface]

Question

I know I risk a thrashing but I feel like I'm going in circles on this. In order to have models available to multiple projects, we have moved the models out to a separate project (a DLL) as a series of interfaces to implement. On of our interfaces has this line:

List<IImportOrderLineModel> Lines { get; set; }

Which links to this interface:

 public interface IImportOrderLineModel
    {
        string OrderNumber { get; set; }
        int? LineNumber { get; set; }
        string ItemId { get; set; }
        string CustomerItemId { get; set; }
        double? Quantity { get; set; }
        decimal? Price { get; set; }
        int? QuantityBackOrdered { get; set; }
        string Comments { get; set; }
        string PickLocation { get; set; }
        string OrderFilled { get; set; }
        string HostUom { get; set; }
        string Type { get; set; }
    }

So in code, when I implement the interfaces and set the properties, I am able to set them without any problem. I have created a concrete implementation of both interfaces and filled them accordingly. Now I am trying to do this:

 Parallel.ForEach(orders, order =>
        {
            order.Lines = items.FindAll(x => x.OrderNumber == order.OrderNumber).ToList();

        });

In order to combine all the items into the corresponding Lines property for the orders. Now this code worked fine when the models were included in the code, but is now broken when we moved them out as interfaces and implement them in a concrete method. The error is:

Cannot implicitly convert type 'System.Collection.Generic.List<ImportOrderLineModel>' to 'System.Collections.Generic.List<IImportOrderLineModel>'

I have read about co-variance and the apple is not a fruit answers that have been given elsewhere, but this is a new one to me. I have tried to use the following on the interface:

 public interface IImportOrderLineModel<out ImportOrderLineModel> where ImportOrderLineModel : class, IImportOrderModel

But doing so generates an error on the line in the other interface:

List<IImportOrderLineModel> Lines { get; set; }

Using generic type 'IImportsOrderLineModel' requires 1 type arguments

How can I get past this?

Answer 1

Looks like this may work:

order.Lines = items.FindAll(x => x.OrderNumber == order.OrderNumber)
                   .Cast<IImportOrderLineModel>()
                   .ToList();

Answer 2

The accepted answer appears to be correct, but there are better ways to solve this problem. The preferred solution would be:

IEnumerable<IImportOrderLineModel> lines = 
  items.Where(x => x.OrderNumber == order.OrderNumber);
order.Lines = lines.ToList();

This solution eliminates two issues with the solution in Renat's answer:

• You start with a list and end with a list, so you'll need to create at least one new list. But FindAll creates another list, unnecessarily. That's extra work, extra collection pressure, and so on, in exchange for no benefit.
• The Cast<IImportOrderLineModel> operation similarly creates a new enumerator, but it is unnecessary; you can always simply do a covariant reference conversion for free to IEnumerable<IImportOrderLineModel> .