C# - Cannot implicitly convert type List<Product> to List<IProduct>

Question

I have a project with all my Interface definitions: RivWorks.Interfaces
I have a project where I define concrete implmentations: RivWorks.DTO

I've done this hundreds of times before but for some reason I am getting this error now:

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

Interface definition (shortened):

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; }         ...     } } 

Concrete class definition (shortened):

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     } } 

In another class I have:

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>();     ... 

Any ideas why this might be happening? (And I am sure it is something trivially simple!)

Answer 1

Yep it's a covariance limitation in C#. You can't convert a list of one type to a list of another.

Instead of:

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

You have to do this

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

Eric Lippert explains why they implemented it this way: http://blogs.msdn.com/ericlippert/archive/tags/Covariance+and+Contravariance/default.aspx

(And why it is different than working with arrays of items).