C# - 无法将类型List <product>隐式转换为List <iproduct> </iproduct> </product>

时间:2010-04-14 20:20:27

标签: c# interface compiler-errors

我有一个包含所有界面定义的项目:RivWorks.Interfaces
我有一个项目,我定义具体的实施:RivWorks.DTO

之前我已经完成了数百次,但出于某种原因我现在收到了这个错误:

  

无法隐式转换类型'System.Collections.Generic.List&lt; RivWorks.DTO.Product&gt;'到'System.Collections.Generic.List&lt; RivWorks.Interfaces.DataContracts.IProduct&gt;'

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>();
    ...

为什么会发生这种情况的任何想法? (我相信这很简单!)

6 个答案:

答案 0 :(得分:98)

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。

而不是:

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解释了为什么他们这样实现它: http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx

(为什么它与使用项目数组不同)。

答案 1 :(得分:35)

你做不到。如果您有List<IProduct>,则可以在其中添加任何 IProduct。因此,如果您有Product2实现IProduct,则可以将其放入列表中。但原始列表创建为List<Product>,因此使用该列表的任何人都只希望列表中包含Product类型而非Product2的对象。

在.NET 4.0中,他们为界面添加了协方差和逆变,因此您可以将IEnumerable<Product>转换为IEnumerable<IProduct>。但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“把东西拿出来”。

答案 2 :(得分:5)

正如一句话:C {4.0中添加了Covariance and Contravariance in Generics

答案 3 :(得分:4)

好吧,你可以用它!

        class A {}
        class B : A {}
        ...
        List<B> b = new List<B>();
        ...
        List<A> a = new List<A>(b.ToArray());

现在,直接解决,

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

答案 4 :(得分:2)

这是一个如何做的小例子。

(newtype) new = (newtype) in;
String Url = new.appropriatemethod();

答案 5 :(得分:-3)

请改为尝试:

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