C# - 无法将List <Product>类型隐式转换为List <IProduct>

Question

我有一个包含所有接口定义
的项目:RivWorks.Interfaces 我有一个项目,我定义了具体的实现:RivWorks.DTO

我之前已经完成了数百次但由于某种原因我现在收到此错误:

无法将类型'System.Collections.Generic.List <RivWorks.DTO.Product>'隐式转换为'System.Collections.Generic.List <RivWorks.Interfaces.DataContracts.IProduct>'

接口定义(缩写):

namespace RivWorks.Interfaces.DataContracts
{
    public interface IProduct
    {
        [XmlElement]
        [DataMember(Name = "ID", Order = 0)]
        Guid ProductID { get; set; }
        [XmlElement]
        [DataMember(Name = "altID", Order = 1)]
        long alternateProductID { get; set; }
        [XmlElement]
        [DataMember(Name = "CompanyId", Order = 2)]
        Guid CompanyId { get; set; }
        ...
    }
}

具体类定义(缩写):

namespace RivWorks.DTO
{
    [DataContract(Name = "Product", Namespace = "http://rivworks.com/DataContracts/2009/01/15")]
    public class Product : IProduct
    {
        #region Constructors
        public Product() { }
        public Product(Guid ProductID)
        {
            Initialize(ProductID);
        }
        public Product(string SKU, Guid CompanyID)
        {
            using (RivEntities _dbRiv = new RivWorksStore(stores.RivConnString).NegotiationEntities())
            {
                model.Product rivProduct = _dbRiv.Product.Where(a => a.SKU == SKU && a.Company.CompanyId == CompanyID).FirstOrDefault();
                if (rivProduct != null)
                    Initialize(rivProduct.ProductId);
            }
        }
        #endregion

        #region Private Methods
        private void Initialize(Guid ProductID)
        {
            using (RivEntities _dbRiv = new RivWorksStore(stores.RivConnString).NegotiationEntities())
            {
                var localProduct = _dbRiv.Product.Include("Company").Where(a => a.ProductId == ProductID).FirstOrDefault();
                if (localProduct != null)
                {
                    var companyDetails = _dbRiv.vwCompanyDetails.Where(a => a.CompanyId == localProduct.Company.CompanyId).FirstOrDefault();
                    if (companyDetails != null)
                    {
                        if (localProduct.alternateProductID != null && localProduct.alternateProductID > 0)
                        {
                            using (FeedsEntities _dbFeed = new FeedStoreReadOnly(stores.FeedConnString).ReadOnlyEntities())
                            {
                                var feedProduct = _dbFeed.AutoWithImage.Where(a => a.ClientID == companyDetails.ClientID && a.AutoID == localProduct.alternateProductID).FirstOrDefault();
                                if (companyDetails.useZeroGspPath.Value || feedProduct.GuaranteedSalePrice > 0)     // kab: 2010.04.07 - new rules...
                                    PopulateProduct(feedProduct, localProduct, companyDetails);
                            }
                        }
                        else
                        {
                            if (companyDetails.useZeroGspPath.Value || localProduct.LowestPrice > 0)                // kab: 2010.04.07 - new rules...
                                PopulateProduct(localProduct, companyDetails);
                        }
                    }
                }
            }
        }
        private void PopulateProduct(RivWorks.Model.Entities.Product product, RivWorks.Model.Entities.vwCompanyDetails RivCompany)
        {
            this.ProductID = product.ProductId;
            if (product.alternateProductID != null)
                this.alternateProductID = product.alternateProductID.Value;
            this.BackgroundColor = product.BackgroundColor;
            ...
        }
        private void PopulateProduct(RivWorks.Model.Entities.AutoWithImage feedProduct, RivWorks.Model.Entities.Product rivProduct, RivWorks.Model.Entities.vwCompanyDetails RivCompany)
        {
            this.alternateProductID = feedProduct.AutoID;
            this.BackgroundColor = Helpers.Product.GetCorrectValue(RivCompany.defaultBackgroundColor, rivProduct.BackgroundColor);
            ...
        }
        #endregion

        #region IProduct Members
        public Guid ProductID { get; set; }
        public long alternateProductID { get; set; }
        public Guid CompanyId { get; set; }
        ...
        #endregion
    }
}

在另一堂课中,我有:

using dto = RivWorks.DTO;
using contracts = RivWorks.Interfaces.DataContracts;
...
public static List<contracts.IProduct> Get(Guid companyID)
{
    List<contracts.IProduct> myList = new List<dto.Product>();
    ...

任何想法为什么会这样？(我相信这很简单!)

Answer 1

是的,它是C#中的协方差限制.您无法将一种类型的列表转换为另一种类型的列表.

代替:

List<contracts.IProduct> myList = new List<dto.Product>();

你必须这样做

List<contracts.IProduct> myList = new List<contracts.IProduct>();

myList.Add(new dto.Product());

Eric Lippert解释了为什么他们以这种方式实现它:http: //blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx

(以及为什么它与使用项目数组不同).

Answer 2

你不能这样做.如果你有List<IProduct>,你可以把任何 IProduct在它.因此,如果你有一个Product2实现,IProduct你可以把它放在列表中.但原始列表创建为List<Product>,因此使用该列表的任何人都只期望类型的对象Product,而不是Product2列表中的对象.

在.NET 4.0中,他们为接口添加了协方差和逆变,因此您可以转换IEnumerable<Product>为IEnumerable<IProduct>.但是这仍然不适用于列表,因为列表界面允许你"把东西放进去"和"把东西拿出去".

Answer 3

正如一句话:C#4.0中添加了泛型中的协方差和逆变.