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C# - 无法将类型 List 隐式转换为 List

转载 作者:IT王子 更新时间:2023-10-29 03:38:47 26 4
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我有一个包含所有接口(interface)定义的项目:RivWorks.Interfaces
我有一个项目,我在其中定义了具体的实现:RivWorks.DTO

我以前做过数百次,但出于某种原因,我现在收到此错误:

Cannot implicitly convert type 'System.Collections.Generic.List<RivWorks.DTO.Product>' to 'System.Collections.Generic.List<RivWorks.Interfaces.DataContracts.IProduct>'

接口(interface)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
}
}

在另一个类(class)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>();
...

知道为什么会发生这种情况吗? (而且我确信这是非常简单的事情!)

最佳答案

是的,这是 C# 中的协变限制。您不能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 解释了他们以这种方式实现的原因: http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx ( archived )

(以及为什么它与处理项目数组不同)。

关于C# - 无法将类型 List<Product> 隐式转换为 List<IProduct>,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/2640738/

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